Nested panels

[awvwgk]

Is your feature request related to a problem? Please describe.

I'm trying to migrate a website to sphinx and sphinx-panels has been incredible helpful so far to create the bootstrap layouts we have previously written directly in HTML/CSS. Now I'm trying to nest columns inside a grid layout as described in the bootstrap documentation here, but without success to far. Nested panels are not described in the sphinx-panels documentation, therefore this seems more like a feature request than an actual bug report to me.

The following myst markdown

````{panels}
:column: col-lg-12

```{panels}
First
---
Second
```

---


```{panels}
Third
---
Forth
```

````

Currently yields separate and ill-formatted columns:

Describe the solution you'd like

Eventually, I want to produce a layout like this, using two columns and cards with headers inside the columns for the individual entries:

Using nested panels seems like the easiest way to make this work.

Describe alternatives you've considered

Trying to build the layout myself using the .. div:: directives, but this would be the last resort.

Additional context

Conda environment used (sorry for the pip/conda-forge mix):

conda list
_libgcc_mutex             0.1                 conda_forge    conda-forge
_openmp_mutex             4.5                       1_gnu    conda-forge
ablog                     0.10.13                  pypi_0    pypi
alabaster                 0.7.12                     py_0    conda-forge
attrs                     20.3.0                   pypi_0    pypi
babel                     2.9.0              pyhd3deb0d_0    conda-forge
beautifulsoup4            4.9.3              pyhb0f4dca_0    conda-forge
brotlipy                  0.7.0           py39h3811e60_1001    conda-forge
ca-certificates           2020.12.5            ha878542_0    conda-forge
certifi                   2020.12.5        py39hf3d152e_1    conda-forge
cffi                      1.14.5           py39he32792d_0    conda-forge
chardet                   4.0.0            py39hf3d152e_1    conda-forge
click                     7.1.2              pyh9f0ad1d_0    conda-forge
colorama                  0.4.4              pyh9f0ad1d_0    conda-forge
cryptography              3.4.4            py39h95dcef6_0    conda-forge
docutils                  0.16             py39hf3d152e_3    conda-forge
feedgen                   0.9.0                    pypi_0    pypi
idna                      2.10               pyh9f0ad1d_0    conda-forge
imagesize                 1.2.0                      py_0    conda-forge
invoke                    1.5.0                    pypi_0    pypi
jinja2                    2.11.3             pyh44b312d_0    conda-forge
latexcodec                2.0.1                    pypi_0    pypi
ld_impl_linux-64          2.35.1               hea4e1c9_2    conda-forge
libffi                    3.3                  h58526e2_2    conda-forge
libgcc-ng                 9.3.0               h2828fa1_18    conda-forge
libgomp                   9.3.0               h2828fa1_18    conda-forge
libstdcxx-ng              9.3.0               h6de172a_18    conda-forge
lxml                      4.6.2                    pypi_0    pypi
markdown-it-py            0.6.2                    pypi_0    pypi
markupsafe                1.1.1            py39h3811e60_3    conda-forge
mdit-py-plugins           0.2.5                    pypi_0    pypi
myst-parser               0.13.5                   pypi_0    pypi
ncurses                   6.2                  h58526e2_4    conda-forge
openssl                   1.1.1k               h7f98852_0    conda-forge
packaging                 20.9               pyh44b312d_0    conda-forge
pip                       21.0.1             pyhd8ed1ab_0    conda-forge
pybtex                    0.24.0                   pypi_0    pypi
pybtex-docutils           1.0.0                    pypi_0    pypi
pycparser                 2.20               pyh9f0ad1d_2    conda-forge
pydata-sphinx-theme       0.6.3                    pypi_0    pypi
pygments                  2.8.0              pyhd8ed1ab_0    conda-forge
pyopenssl                 20.0.1             pyhd8ed1ab_0    conda-forge
pyparsing                 2.4.7              pyh9f0ad1d_0    conda-forge
pysocks                   1.7.1            py39hf3d152e_3    conda-forge
python                    3.9.2           hffdb5ce_0_cpython    conda-forge
python-dateutil           2.8.1                    pypi_0    pypi
python_abi                3.9                      1_cp39    conda-forge
pytz                      2021.1             pyhd8ed1ab_0    conda-forge
pyyaml                    5.4.1                    pypi_0    pypi
readline                  8.0                  he28a2e2_2    conda-forge
requests                  2.25.1             pyhd3deb0d_0    conda-forge
setuptools                49.6.0           py39hf3d152e_3    conda-forge
six                       1.15.0             pyh9f0ad1d_0    conda-forge
snowballstemmer           2.1.0              pyhd8ed1ab_0    conda-forge
soupsieve                 2.2                      pypi_0    pypi
sphinx                    3.5.1              pyhd8ed1ab_0    conda-forge
sphinx-book-theme         0.1.0                    pypi_0    pypi
sphinx-copybutton         0.3.1                    pypi_0    pypi
sphinx-panels             0.5.2                    pypi_0    pypi
sphinx-togglebutton       0.2.3                    pypi_0    pypi
sphinx_rtd_theme          0.5.1              pyhd3deb0d_0    conda-forge
sphinxcontrib-applehelp   1.0.2                      py_0    conda-forge
sphinxcontrib-bibtex      2.2.0                    pypi_0    pypi
sphinxcontrib-devhelp     1.0.2                      py_0    conda-forge
sphinxcontrib-htmlhelp    1.0.3                      py_0    conda-forge
sphinxcontrib-jsmath      1.0.1                      py_0    conda-forge
sphinxcontrib-qthelp      1.0.3                      py_0    conda-forge
sphinxcontrib-serializinghtml 1.1.4                      py_0    conda-forge
sqlite                    3.34.0               h74cdb3f_0    conda-forge
tk                        8.6.10               hed695b0_1    conda-forge
tzdata                    2021a                he74cb21_0    conda-forge
urllib3                   1.26.3             pyhd8ed1ab_0    conda-forge
watchdog                  2.0.2                    pypi_0    pypi
wheel                     0.36.2             pyhd3deb0d_0    conda-forge
xz                        5.2.5                h516909a_1    conda-forge
yaml                      0.2.5                h516909a_0    conda-forge
zlib                      1.2.11            h516909a_1010    conda-forge

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[chrisjsewell]

This will essentially be closed by executablebooks/sphinx-design#6