docs: release notes for 0.5.0

[christianhuening]

The remaining blocker before tagging 0.5.0.

[Unreleased] documented a @tiptap/core 2.27.3 security backport that
main no longer ships (it's on 3.31.3), plus a caveat that scanners "will still
be red" which is no longer true. release.yaml lifts that section verbatim
onto the GitHub Release page, so renaming the heading would have published a
false security claim as the release headline.

Covers #14–#21 in the voice of the 0.3.0/0.4.0 entries.

Notable

• ### Operators — read before upgrading (precedent: 0.2.0) carries the
  four things an upgrade actually asks of someone — no migrations and clean
  rollback; reload open tabs; a stale 0.4.0 tab strips the link titles this
  release fixes; comments anchored to the first character of a document lose
  their highlight.
• Corrects 0.4.0's notes. They said code blocks, Mermaid and Excalidraw
  break out into the room in wide mode. Only tables did — the selectors were
  dead, which is one of the four bugs fixed here.

Two claims I had to correct while writing

Both were in my own draft and both were wrong:

• "20 sibling packages" → web/package.json has 16 @tiptap/* entries.
  Replaced with "the whole @tiptap/* set".
• "Tiptap 2 had been unmaintained" → false. There is a live v2-latest
  dist-tag at 2.27.3, published 2026-09-04. It is maintenance-only, which is a
  weaker and more accurate argument for migrating.

Verified every ## [x.y.z] heading has a matching compare link and vice versa;
added the [0.5.0] link and repointed [Unreleased].

After this merges

Only the tag is left — no version files to bump (Chart.yaml, Cargo.toml,
web/package.json are inert 0.0.0, forced from the tag):

git tag -a v0.5.0 -m "knot 0.5.0

<lede paragraph from the 0.5.0 section>

See CHANGELOG.md for the full notes."
git push origin v0.5.0

Still worth running first, since this batch never triggered it:
gh workflow run helm-upgrade.yaml --ref main.

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