Support adding intersphinx links

[tremble]

Following the release of antsibull-docs 1.7.0 we now have access to --intersphinx when initializing sphinx.
ansible-community/antsibull-docs#44

This means that we can automatically generate links between multiple docs sites

[tremble]

Example usage ansible-collections/community.aws#1554
Example run https://github.com/tremble/community.aws/actions/runs/3205260754

[tremble]

@briantist @felixfontein

[felixfontein]

The CI errors should go away once #55 is merged.

[briantist]

Thank your for submitting this! If possible I'd really like to see some tests, if only for the action which has a set of tests already.

[tremble]

@felixfontein done

[briantist]

@tremble thank you for adding this feature!
@felixfontein thank you for reviewing and adding the relevant feature to antsibull-docs!