RFR: 8357000: Write overview documentation for start of release changes [v4]

[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 incrementally with two additional commits since the last revision:

 - Update description of start of release changes.
 - Move file up a directory.

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/25317/files
  - new: https://git.openjdk.org/jdk/pull/25317/files/1e221090..87b06f84

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=25317&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=25317&range=02-03

  Stats: 238 lines in 3 files changed: 195 ins; 43 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