Require targets to explicitly import pigweed variable definitions. This
moves out some necessary logic from the BUILDCONFIG.gn file.
Change-Id: If9c9fe5a680cc9d897a0e8a55889873be618ed84
Adds target docs for the `docs` and `host` builds. Also includes a few
other documentation updates.
Change-Id: Icc973d6d51707c79b7c506d89c8975023e274ef2