Convert collection links from raw HTML to directive

[felixfontein]

Currently includes #195, #198, and #199. Will rebase once these are merged.

Converts the inline HTML for collection links to a custom directive. The external links are a custom nodes class inherited from docutils's reference with a custom HTML generator; for other builders, the reference generator provided by that builder is used (so in LaTeX it is a regular hyperlink reference). The "Communication" button though needs a Sphinx pending_xref node (to resolve the reference), which improves on the previous version by not relying on implementation details, but at the same time is worse than before because pending_xref is replaced by a plain ref (Sphinx doesn't allow to customize this), which rendered to HTML neither has target="_blank" nor aria-role="button". (Aside from that, it looks and behaves like a button, though.)

The HTML output didn't change (except the missing attributes for the "Communication" button, which you cannot see here):

The LaTeX output is now as expected - the first four buttons are hyperlinks, the last one an internal reference in the PDF file:

[felixfontein]

You can see the HTML result on:

• https://ansible.fontein.de/collections/community/general/index.html#description
• https://ansible.fontein.de/collections/community/general/aerospike_migrations_module.html#collection-links

[oraNod]

LGTM. Some really minor nits on wording. I couldn't find any faults or optimizations for the code side but I did look and it all seems fine. I think this is really nice work @felixfontein

[felixfontein]

@oraNod thanks a lot for reviewing this and improving the formulations :)

[oraNod]

@oraNod thanks a lot for reviewing this and improving the formulations :)

you're welcome. thanks for all your work!