Generated migration guide would benefit from automated cleanup

[alice-i-cecile]

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 #1191 (comment)

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

[alice-i-cecile]

We should be able to do a basic formatting pass on the contents of the migration guide before generating them :)

Release notes don't need this step: the starting content needs to be completely rewritten anyways.