ci: github: doc: Add a GH workflow to test build the docs

This github workflow will build the html docs on a pull request or push.
This is similar to the current ci-tools 'Documentation' check.  One
difference is this version produces a GH artifact of the html docs
instead of posting it to S3.  The artifact is a tarball that is than
zip'd (not gzip).

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>