This change adds documentation for the pw_docgen module describing how
to use it to build documentation for Pigweed projects.
Change-Id: Ia8bb57d26b96f748adc021cc86f0a16865611295
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