Add HTML docs and expand docs

[lagru]

Using Sphinx, Furo theme and myst parser for now. Might want to use this with readthedocs for the versioning support?

Release note

For maintainers and optionally contributors, please refer to changelist's README on how to document this PR for the release notes.

Update documentation and add a glossary with a few central terms.
{label="documentation"}

Setup HTML documentation on Read the Docs.
{label="documentation"}