This commit is intended to improve new user's first-interaction with the Sphinx site:
- Make page header icon/text smaller and not capitalized
- Give min-width to left sidebar (it was getting too small at certain window sizes)
- Replace features list on landing page with admonition boxes, with adaptive layout
- Add landing page "used by" section
- Slightly restructure the Extension section, into Tutorials and How-tos
- Add code to `conf.py` to write HTML write redirect pages for moved documents
- Improve support page, by adding link to Stackoverflow, GH discussion and ReadtheDocs,
and remove defunct link to libera chat and matplotlib tutorial
