Add --intersphinx option to sphinx-init subcommand

[felixfontein]

Fixes #35.

[tremble]

Just tested this locally using:

antsibull-docs sphinx-init --use-current amazon.aws --dest-dir /tmp/docs/amazon.aws/ --intersphinx ansible_devel:https://docs.ansible.com/ansible-core/devel/ --intersphinx community_aws:https://ansible-collections.github.io/community.aws/branch/main/

and

antsibull-docs sphinx-init --use-current community.aws --dest-dir /tmp/docs/community.aws/ --intersphinx ansible_devel:https://docs.ansible.com/ansible-core/devel/ --intersphinx amazon_aws:https://ansible-collections.github.io/amazon.aws/branch/main/

Worked as expected and resulted in the relevant cross-documentation links being generated (deliberate choice to override ansible_devel with the ansible-core docs as this forces amazon.aws/community.aws docs to reference the collection docs rather than possibly pulling links from modules docs in the "ansible" distribution)

[felixfontein]

@tremble @samccann thanks for reviewing and testing!