Add external links to the documentation and update Documenter compat

[kunzaatko]

• fix(docs): Update compat bound for Documenter and dev the package
• feat(docs): Add the DocumenterInterLinks package for external documentation links
• feat(docs): Use @extref for external references
• fix(docs): Add warnonly=[:missing_docs]

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.51%. Comparing base (7d0f38c) to head (2b90223).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #322   +/-   ##
=======================================
  Coverage   73.51%   73.51%           
=======================================
  Files           5        5           
  Lines         540      540           
=======================================
  Hits          397      397           
  Misses        143      143           

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