Floating comment threads: Alternate rendering and UI for blockquote discussions

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Is your feature request related to a problem? Please describe.

Highlight -> comment threads are a common staple in collaboration software today, and the lack of this feature is an early blocker for most open source markdown tooling. This has been discussed some elsewhere but warrants a particularized issue thread IMO

Describe the solution you'd like

I like the idea in #2879 (comment) of using in-doc nested blockquotes as the fundamental data structure, but I'd suggest:

1. Adding an annotation like [style=floating] and [references=^1] so that something like ==comment target[^1]== could be an unambiguous target and the floating sidebar style could be selected by an editor
2. Adding an easy "highlight and comment" affordance that handles the annotations
3. Auto-collapse comment threads in the editor view.

Not entirely sure of the implementation details but [style=floating] could be a generic "block" annotation & the rendered sidebar would just be markdown with some special css. ideally I think the bubbles could even optionally annotate raw editor view.

Describe alternatives you've considered

There is much discussion of alternatives in #2879. While I'm a fan of the critic markup as mentioned in the thread it does not seem suitable for longform sidebar discussion popularized by google docs and the like.

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One key problem with this idea is that hedgedoc's dual-pane design doesn't lend itself as neatly to the UX of other editors. Thinking about it harder, this kind of feature may only make sense if hedgedoc is interested in implementing some unified live preview view like stackedit, or editable-rendered view