Set up a documentation tool

- use sphinx to document the developed package
- create nox session "docs" to build the docs

docs/index.rst

@@ -0,0 +1,25 @@
+lalib - A Python library to study linear algebra
+================================================
+
+.. toctree::
+   :hidden:
+   :maxdepth: 1
+
+   license
+   reference
+
+
+The goal of this package is to provide
+a library written in pure `Python`_ (incl. the `standard library`_)
+to learn about linear algebra by reading and writing code.
+
+
+Prerequisites
+-------------
+
+Python 3.9 or newer is needed.
+The package depends only on core Python (incl. the standard library).
+
+
+.. _standard library: https://docs.python.org/3/library/index.html
+.. _python: https://docs.python.org/3/