docs: Update Sphinx; remove Markdown; add CSS

This completes the update of Sphinx and RST by removing the legacy
Markdown support. It also adds custom CSS skeleton.

Change-Id: I398a21208772cf2d2dd32dee0adac768630dd575
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/40621
Commit-Queue: Keir Mierle <keir@google.com>
Pigweed-Auto-Submit: Keir Mierle <keir@google.com>
Reviewed-by: Anthony DiGirolamo <tonymd@google.com>
Reviewed-by: Ewout van Bekkum <ewout@google.com>