Generated migration guide would benefit from automated cleanup

> Bit of a nit, but I'm not a fan of all the white space inconsistency in the migration guide. How easy would it be to call `str::trim` on the file contents, then write an extra `\n`?

_Originally posted by @BD103 in https://github.com/bevyengine/bevy-website/issues/1191#issuecomment-2126861114_

> Btw, the first thing I did pretty much every release was capitalize every title correctly and standardize all the before/after code blocks to use // bevy 0.13 / // bevy 0.14 people use a bunch of different ways to do that but it's pretty easy to just do a search and replace. Although now you'll need to do a search replace in the folder I guess

- @IceSentry

The issue does not name a file or test; first locate the code that generates the migration guide and inspect its generated output. Verify the requested whitespace cleanup, including trimming file contents and ending with one newline, along with the mentioned title and before/after code-block standardization. Done means regenerated guides use the agreed formatting consistently.

Written by the indexing model from the issue text.