docs(releases): curate the 17.4.0 release notes (#17363)

* docs(releases): curate the 17.4.0 release notes

Replace the machine draft on content/docs/releases/v17/17-4.mdx with curated
release notes in the 17.3.0 shape: Highlights, themed breaking-change
subsections with a Migration note each, grouped new capabilities, a
notable-fixes passage in place of the per-package count table, the two objectui
pin moves, and a derived upgrade checklist whose every line is marked
not exercised.

Retire the "machine draft" notices on the v17 index and record that the 17.4.0
checklist is derived, not walked.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016gFSzdPX4324QMRKCjccTc

* docs(releases): reflow two paragraphs and drop a measured claim

No line-initial issue reference can be read as an ATX heading, and the
checklist's pin-move step no longer carries the word "measured" — nobody has
walked 17.3.0 to 17.4.0.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016gFSzdPX4324QMRKCjccTc

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Co-authored-by: Claude <noreply@anthropic.com>

Author: os-sales