Replaces the contents of the Pigweed documentation landing page with
the README.md inlined and converted to rst.
Change-Id: I6457aaede5ca4c0585af4a00c388278fb14223f5
This change adds a docs directory at the root level of the Pigweed
repository for building documentation for the entirety of Pigweed.
The directory contains a top-level index.rst and conf.py for docgen,
and a pw_doc_gen build target to render the documentation.
Change-Id: Iba981c64662d48a1d23bb7bda9bf0ed5a104873b
