Update documentation

[StijnKas]

One of the main remaining items for V4 is/was documentation. In this PR, I've setup some infra changes to make publishing new documentation pieces easier and also added some documentation to certain classes and methods.

Additionally, with the goal of moving away from the wiki (for various reasons), this PR contains some of the getting started code that used to be in the wiki, directly in the Python docs.

I've also moved and shifted around some of the main init import logic and made certain classes not available top-level. Let's see if that gives us any issues down the line.

[codecov]

Codecov Report

Attention: Patch coverage is 32.14286% with 57 lines in your changes missing coverage. Please review.

Project coverage is 61.33%. Comparing base (399ae8f) to head (80b7689).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
python/pdstools/cli.py	0.00%	36 Missing ⚠️
python/pdstools/utils/show_versions.py	10.00%	18 Missing ⚠️
python/pdstools/decision_analyzer/decision_data.py	0.00%	2 Missing ⚠️
python/pdstools/adm/ADMDatamart.py	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #283      +/-   ##
==========================================
+ Coverage   58.47%   61.33%   +2.85%     
==========================================
  Files          56       56              
  Lines        4918     4717     -201     
==========================================
+ Hits         2876     2893      +17     
+ Misses       2042     1824     -218     

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[StijnKas]

Pretty much closes #278