docs(releases): 17.2.0 and 17.3.0 detail sections, and repair the upgrade entry page (#15322) (#15333)

Two releases shipped with no upgrade guidance, and the customer's entry page
reads a breaking upgrade as a tag swap.

content/docs/upgrading.mdx
- The per-major checklist table listed v17 once, pointing at 17.0.0. A
  deployment on 17.1.0 got no pointer to either release it has to cross. The
  section becomes "Per-release specifics" and carries a row per v17 release.
- The page's framing ("move the tag, restart") plus the metadata-compatibility
  Callout read together as "a minor is a no-op". A second Callout scopes the
  first: the major boundary promises metadata compatibility and nothing else.
  The two-upgrades-two-clocks argument is untouched.

content/docs/releases/v17.mdx
- New "What's new in 17.2.0" and "What's new in 17.3.0" detail sections, in the
  shape 17.0.0/17.1.0 establish. Content is reused from the packages' own
  CHANGELOG entries rather than paraphrased; every entry is cited by changeset
  hash.
- Release-status blockquote now says 17.3.0 is current, and names what makes
  17.3.0 a minor by number only.
- "Upgrade checklist" and "References" are promoted from h3 to h2 so they stay
  page-level sections after two more release sections are appended. No published
  prose in the 17.0.0 / 17.1.0 sections is changed; the #upgrade-checklist,
  #1700 and #1710 anchors are preserved.

The consolidated 17.1.0 -> 17.3.0 upgrade checklist is deliberately NOT in this
commit: it is blocked on hotcrm#1576's documentation-first upgrade log, and a
checklist step nobody has run is worse than a missing one.


Claude-Session: https://claude.ai/code/session_01UHvF5hyiZjnCyExFnfQB8m

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

Author: hotlong