RFR: 8356689: Make HotSpot Style Guide change process more prominent

[Kim Barrett]

Please review this change to the HotSpot Style Guide. It moves the description
of the style-guide change process to near the beginning of the document,
making it more prominent.

I propose this is an editorial change, as it just moves some text from one
place to another.  As such, the normal HotSpot PR process applies.

The updated .html file was generated using `make update-build-docs`.

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Commit messages:
 - move change process description earlier

Changes: https://git.openjdk.org/jdk/pull/25169/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25169&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8356689
  Stats: 70 lines in 2 files changed: 35 ins; 31 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/25169.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25169/head:pull/25169

PR: https://git.openjdk.org/jdk/pull/25169