Fix the documentation building.

 * Add the missing `.readthedocs.yml` file.
 * Use hdl organization's hdlparse fork.
 * Add missing `third_party/make-env` submodule.
 * Revert the `docs/Makefile` to match skywater-pdk's version.

Signed-off-by: Tim 'mithro' Ansell <tansell@google.com>