docs: Unify READMEs in the documentation and the source code
This merges the old README.md into the docs/README.rst. And creates a link README.rst into the repository top-level.
The main docs/README.rst was only visible from the docs.s.o site. Now it's seen both in the git clone and the docs.
Test Plan
make docs is happy (some warnings but it's local, they won't be warnings when built from swh-docs).
Migrated from D5339 (view on Phabricator)