Add docstrings and documentation site

[GeertD]

Added the following:

• Solution: Google docstrings, both for methods as attributes.
• documentation site: using Zensical (better alternative to MkDocs), added a few start pages and API reference, settings, see zensical.toml file.
• API reference: uses mkdocstrings-python module, works with Zensical, lots of configuration, settings: see zensical.toml file.

Documentation site is hosted on Github Pages, on my fork.

The documentation is deployed automatically by .github/workflows/docs.yml.
Documentation is recreated and deployed with every new commit.

Ran pytest and code still OK.

Regards,

Geert

[GeertD]

Some details about Zensical and alternatives:

• Sphinx: steep learning curve, uses RestructuredText instead of Markdown, harder to read;
• MkDocs: started with this one, easy to use, integrates well with mkdocstrings-python (API reference), but looks like this recently is no longer supported and split up;
• Zensical: complete rewrite of MkDocs by the guys of the Material theme for MkDocs (most popular theme), good documentation, good reviews, looks like a stable project.

Zensical pros:

• very good documentation;
• lots of configuration possible (customization);
• incremental build (only updates pages affected);
• works well with mkdocstrings-python;
• great features: light/dark theme, color themes, search, navigation, ... .

Mkdocstrings-python pros:

• easy plugin for Zensical;
• good documentation;
• lots of configuration.