deposit.api: Add a basic api page to https://deposit.s.o/1
This makes the link https://d.s.o/1 referenced from the getting-started [1] actually resolves to a page and not a 404.
Note: This also improve some sentence phrasing in the main homepage and some module docstrings
Related to #2140 (closed)
Test Plan
tox
Migrated from D2401 (view on Phabricator)