[WIP] Add new semantic markup support to antsibull-docs

[felixfontein]

Implements antsibull-docs part of ansible/ansible#73137. See also ansible/ansible#74937.

(For now contains #272 so I have less chaos locally :) )

[felixfontein]

I updated this PR to the latest specification in https://hackmd.io/VjN60QSoRSSeRfvGmOH1lQ?both, with the small difference that [] can be used in option names and return value names, and there is a missing: prefix (as opposed to FQCN#type:) to indicate that this is not an existing option/return value name (for example to reference something that has been removed in a previous release). I've also updated #373 and re-built my docsite (https://ansible.fontein.de) with this. The references are now also linking to the option/return value documentation. Examples:

• https://ansible.fontein.de/collections/community/dns/docsite/hosttech_guide.html#querying-dns-records-and-record-sets (note the links in the sentence below the first example)
• https://ansible.fontein.de/collections/community/dns/docsite/hosttech_guide.html#bulk-synchronization-of-dns-record-sets (note the links in the sentence below the first example; also shows the [] notation)
• https://ansible.fontein.de/collections/community/dns/hetzner_dns_record_module.html#parameters (note that record, zone_name, and prefix can be clicked)
• https://ansible.fontein.de/collections/community/dns/hetzner_dns_record_sets_module.html#parameter-record_sets (shows [] and . notation)
• https://ansible.fontein.de/collections/community/dns/wait_for_txt_module.html#parameter-records (shows [] and . notation)

(Currently includes #373.)

[felixfontein]

This will be continued in ansible-community/antsibull-docs#4 since antsibull-docs is removed from this package.