0.13 migration: Add section for entity niche optimization

Migration guide generator didn't catch

https://github.com/bevyengine/bevy/pull/9907

Because it's unlabeled and the formatting of the description isn't correct.

Unfortunately, the migration section in the description doesn't actually offer any advice for migration, so we'll need to write some.

I'm not actually sure how people can migrate this, other than recreating their scenes. It might be possible to create a small program/script/abuse awk to bit-twiddle entity ids in scene files.

See https://github.com/bevyengine/bevy/pull/11289 for an example of fixing a scene file.

More context: https://discord.com/channels/691052431525675048/1209244908372627507

Start by reading the migration guide generator context and PR 9907 to understand why this change was missed. Use PR 11289 as the example for correcting a scene file, then determine and document a practical migration path for entity niche optimization, including whether recreating scenes or transforming scene files is required.

Written by the indexing model from the issue text.