Improve cross-linking in docs and add linkcheck

This MR ...

Can this MR be accepted?

• Implementation ready
  • use intersphinx to crosslink to other projects
  • use apidoc.typehints for more crosslinks
  • hide module names and reduces the places in which fully-qualified names are shown in the docs
  • drop --separate option: now have API for one module all on the same HTML page (makes jumping easier)
  • add linkcheck to the build routine
  • fix bad links throughout the docs
  • updates dependency specifications
  • migrate old-style API links to intersphinx-based crosslinks
  • reduce .nitpick-ignore entries and fix corresponding references
• Tests added or adjusted
• Documentation extended or updated

• Code quality
  • Changes follow coding guidelines
  • Checked code coverage on new and adjusted code

• Ready for merging
  • Pipeline passes without warnings
  • History cleaned-up or squash option set
  • Changelog entry added
  • Version number bumped
  • Reviewed & approved

Related MRs and issues

utopya#11 (closed), utopya!8 (merged)