Group hints together outside of document's body

[heiwiper]

Description

This PR moves all the hints that were previously children of the document's body under a new element which is sibling of body.

Fixes #3261

Discussion

There was a discussion in #3261 about whether the new element that is the parent of hints should be a shadow root in order to encapsulate the style of hints. For now this PR seems to work just fine, I have inspired mostly from LinkHints extension which unsets styling of the hints parent but also uses a shadow root, so there might be an actual good reason for using encapsulation.

Checklist:

Everything in this checklist is required for each PR. Please do not approve a PR that does not have all of these items.

• Git hygiene:
  • I have pulled from master before submitting this PR
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• I've added the new dependencies as:
  • ASDF dependencies,
  • Git submodules,

    cd /path/to/nyxt/checkout
    git submodule add https://gitlab.common-lisp.net/nyxt/py-configparser _build/py-configparser

  • and Guix dependencies.
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  • I have updated the existing documentation to match my changes.
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[aadcg]

Pushed to master via commits 6f3f346 and 3c40dbb. In the latter commit, I've fixed the changelog entry. Thanks @heiwiper! Amazing work, as always.

but also uses a shadow root, so there might be an actual good reason for using encapsulation.

I think that we need to make the transition to a shadow root at some point, but it requires more work than the major and yet low-effort improvement presented in this PR. The top priority here was to prevent embarrassing bugs such as #3261. See #3285.

Nonetheless, with respect to hint-mode, #3254 is undoubtedly the highest priority.