gbl: export docs and C headers as build artifacts

Adds a new build artifact `gbl-resources-{build_number}.zip`. This
zipfile contains:

- README.md: new top-level readme describing the archive
- docs/: all files in gbl/docs/
- include/: all files in gbl/libefi_types/include

The purpose is to make it easier for developers to develop against GBL.
This makes it so they don't have to go digging through our code to find
the information they need, and more importantly makes it simple to
guarantee they are developing against the exact same ABI versions as
their GBL binary, which was previously surprisingly difficult.

Also does a slight update of a few of the other README files to make
them a little more descriptive of the current state.

Bug: b/501431053
Change-Id: I113be3fae095001e33b5d9232d63049b9c58e0d0