Improve support for external issue references

What does this MR do?

1. Fixes referenced bugs related to extracting references to external issues
2. Cleans up the code - Banzai::Filter::IssueReferenceFilter should not concerned with external issues, because we have Banzai::Filter::ExternalIssueReferenceFilter.
3. Clarifies in the documentation what we support and what we do not support

Are there points in the code the reviewer needs to double check?

General caution, this code was very messy and unintuitive, and it still is.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• [ ] API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #33679 (closed)

Closes #34076 (closed)

Closes #34082 (closed)