Write a HOWTO migrate Codec format version

### Description

The change to changing PFOR encoding to FOR for doc blocks in https://github.com/apache/lucene/pull/12741, required bumping the Codec version from 95 to 99.

Bumping the Codec is not a straightforward process since it involved:
* Forking all modified changed files and bumping the version number
* Moving the original copies of the modified files to backwards_codecs
* Re-generating ForUtil
* \+ changes to [forUtil.gradle](https://github.com/apache/lucene/pull/12741/files#diff-6bfb3ba57326f955b44ec86c40070c4e518bffb45cf6226504eab043d545ffbd) to bump version and add a new task for regenerating backwards_codec
* Forking PostingsFormat subclasses
* Changes to tests:
* Bumping existing Codec unit tests
* Creating RW versions of old Codec and Codec components
* Bumping Codec components where used throughout other tests

We required a few follow-ups to clean up the initial misses:
* https://github.com/apache/lucene/pull/12781
* https://github.com/apache/lucene/pull/12776
* https://github.com/apache/lucene/pull/12908
* https://github.com/apache/lucene/pull/12904

@mikemccand mentioned in https://github.com/apache/lucene/pull/12904#issuecomment-1849951055
> Codec format migrations are so tricky ... I wish we had a HOWTO somewhere.

Since I done the last Codec bump, I can have a look at writing up the documentation so subsequent codec format bumps will go smoother.

Start by reviewing the Codec migration described in PR 12741, its follow-ups, and the changes to forUtil.gradle. Use the listed steps around ForUtil, backwards_codecs, PostingsFormat subclasses, and Codec tests to structure the HOWTO. Done means documenting the migration sequence and the missed follow-up considerations so future Codec bumps are more predictable.

Written by the indexing model from the issue text.