Documentation: Docstrings, Sphinx documentation (#8)

* Improved copilot instructions

* Add missing docstrings, rework existing ones

* Add VSCode settings to git

* Initial readthedocs setup (MkDocs 1)

* Replace MkDocs with Zensical

* Switch to Sphinx documentation tooling

* Add copyright and missing docstrings to Sphinx doc

* Group docs into sub-directories

* Further improve Sphinx docs

* Friendlier introduction and a bit of history

* Remove reading path from README

* src/tests/testapp/apps.py: Copilot stylistic warnings

* Fix remaining CoPilot warnings