Stop using intersphinx

Remove intersphinx from the docs build as it triggers network calls that
occasionally fail, and we don't really use intersphinx (links other
sphinx documents out on the internet)

This also removes the requirement for internet access during docs build.

This causes docs jobs to fail because we error out on warnings.

Change-Id: I71e941e2a639641a662a163c682eb86d51de42fb
Related-Bug: #1368910