* Support Markdown in documents
Uses `myst_parser` to process Markdown files in the Sphinx workflow.
Using that, included the top level `README.md` as the "Overview" in the
rendered docs.
Some tweaks were needed to have `README.md` render correctly in rendered
docs. Still renders okay via GitHub.
Add a makefile target `doc-preview` to serve the locally built docs.
Fix some links in the README file.
Co-authored-by: Sven Marnach <sven@mozilla.com>