Add info about how to write migration guides to contributor book

Yesterday I wanted to write a migration guide for one of my PRs. I didn't know how to, so initially (naturally) I was looking at https://bevy.org/learn/contribute/introduction/. However there is no info about the migration guides creation there, so I had to ask on discord.

It would be nice to have a very brief section in either

- https://bevy.org/learn/contribute/helping-out/opening-pull-requests/
- https://bevy.org/learn/contribute/helping-out/writing-docs/

Start with the contributor-book pages linked in the issue, especially Opening Pull Requests and Writing Docs, and compare their structure and guidance. Add a brief section explaining how to create migration guides in the most appropriate location, then check the rendered page to confirm the information is clear and accessible.

Written by the indexing model from the issue text.