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 can cause docs jobs to fail if the project errors out on
warnings.

Change-Id: Id2b52d22e7c946d75f421a8f731889d5afc34b9d
Related-Bug: #1368910