Add footnotes support

[mejo-]

📝 Summary

• Resolves: Footnote support would be nice #2142
• Resolves: Footnote support collectives#283

Some implementation details:

• In UI, the reference IDs are hidden on purpose. Instead, all references and their corresponding footnotes get a sequential ordinal that is displayed. The footnotes are ordered by their ordinal. This is how Obsidian and others do it as well and it avoids confusion regarding manually used and out-of order numeric references.
• References can be inserted either via [^label] input rule or via Ctrl-Shift-F shortcut. The latter automatically creates a reference ID (the first digit that is not already in use as reference ID).
• Deleting a reference also removes it's corresponding footnote if no other reference to the footnote exists.

🖼️ Screenshots

Editor	with visible container	Markdown

Todo

• Clarify if we want to add a menubar entry to insert a footnote. If so, where do we place it?
• Decide whether we want the visual container with background color or not.

🏁 Checklist

• Code is properly formatted (npm run lint / npm run stylelint / composer run cs:check)
• Sign-off message is added to all commits
• Tests (unit, integration and/or end-to-end) passing and the changes are covered with tests

🤖 AI (if applicable)

• The content of this PR was partly generated using AI tools
• The AI-generated content was reviewed, comprehended and tested by a human

[marcoambrosini]

Very nice! A couple of comments from a quick first pass:

• I would avoid using a container and gray out the text instead
• I would put this in a new add menu in the toolbar, same as the + in the document, where we can access everything from one bucket. And with a search-bar on top of the list