RFR: 8357000: Write overview documentation for start of release changes

[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

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Commit messages:
 - Appease jcheck.
 - JDK-8357000: Write overview documentation for start of release changes

Changes: https://git.openjdk.org/jdk/pull/25317/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25317&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8357000
  Stats: 43 lines in 1 file changed: 43 ins; 0 del; 0 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