doc: enrich documentation frontpage

For now, the content is taken from the README.

docs/index.rst

@@ -1,18 +1,38 @@
-.. datasalad documentation master file, created by
-   sphinx-quickstart on Tue May 14 15:31:26 2024.
-   You can adapt this file completely to your liking, but it should at least
-   contain the root `toctree` directive.
+The `datasalad` documentation
+=============================
 
-Welcome to datasalad's documentation!
-=====================================
+This is a pure-Python library with a collection of utilities for working with
+data in the vicinity of Git and git-annex.  While this is a foundational
+library from and for the `DataLad project <https://datalad.org>`__, its
+implementations are standalone, and are meant to be equally well usable outside
+the DataLad system.
 
-.. toctree::
-   :maxdepth: 2
-   :caption: Contents:
+A focus of this library is efficient communication with subprocesses, such as
+Git or git-annex commands, which read and produce data in some format. The
+library provides utilities to integrate such subprocess in Python algorithms,
+for example, to iteratively amend information in JSON-lines formatted data
+streams that are retrieved in arbitrary chunks over a network connection.
 
+Here is a simple demo how an iterable with inputs can be fed to the ``cat``
+shell command, while reading its output back as a Python iterable.
 
-Provided functionality
-----------------------
+
+.. code-block:: python
+
+    >>> with iter_subproc(
+    ...     ['cat'],
+    ...     inputs=[b'one', b'two', b'three'],
+    ... ) as proc:
+    ...     for chunk in proc:
+    ...         print(chunk)
+    b'one'
+    b'two'
+    b'three'
+
+
+
+Package overview
+----------------
 
 .. currentmodule:: datasalad
 .. autosummary::