RFR: 8357000: Write overview documentation for start of release changes [v3]

[Joe Darcy]

> First attempt to populate "supplementary docs" with a discussion of the start of release changes. For reference on the idea of supplementary docs, see the thread
> 
> "Where to put supplementary docs?"
> https://mail.openjdk.org/pipermail/jdk-dev/2025-April/009975.html

Joe Darcy has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains four additional commits since the last revision:

 - Merge branch 'master' into JDK-8357000
 - Respond to review feedback.
 - Appease jcheck.
 - JDK-8357000: Write overview documentation for start of release changes

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/25317/files
  - new: https://git.openjdk.org/jdk/pull/25317/files/8e10f57b..1e221090

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=25317&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=25317&range=01-02

  Stats: 104522 lines in 1479 files changed: 61995 ins; 28502 del; 14025 mod
  Patch: https://git.openjdk.org/jdk/pull/25317.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25317/head:pull/25317

PR: https://git.openjdk.org/jdk/pull/25317