Automatically generate stubs for release notes and migration guides

[alice-i-cecile]

Each release, we want two lists, scraped from the https://github.com/bevyengine/bevy PRs merged since the last release.

1. Release notes: all PRs in the correct time period that have a Changelog section OR have an C-Enhancement label.
2. Migration guide: all PRs that have a Migration Guide section, OR have a C-Breaking-Change label.

(The labels are there to catch stuff that was missed).

The lists generated should record:

• The PR name, number and link
• The authors of all commits
• The area tags listed (e.g. A-ECS)
• The contents of

These lists should be sorted by area tags (just use the first matching one if there's multiple).

These lists should be manually generated via a command-line method, that accepts two arguments:

1. Whether to generate release note stubs or migration guide stubs.
2. What Bevy version to grab commits after (this should probably stop at the next version's tag, in case we want to do this on a historical basis too?).

Library suggestions

1. clap, an excellent Rust CLI-builder tool.
2. markdown-extract, to parse the sections.
3. octocrab, for scraping Github. Seems to be the most popular / maintained of the current options?

[IceSentry]

If anyone wants to tackle this I would suggest doing something similar to all the generate-* tools we already have

[rparrett]

Here's a janky shell script for the migration guide: https://gist.github.com/rparrett/46e434ba1fec127738fa8932f9e1d482

[IceSentry]

I ended up writing a tiny rust app to generate the migration guide #469. It worked surprisingly well.

[IceSentry]

Oh, I just saw the suggestion to use markdown-extract, that would have been way easier if I used that.

[IceSentry]

I added support to generate release note in #469 it's not complete yet since it doesn't do any filtering or sorting, but it's a start.

[TrialDragon]

I believe this issue was resolved in #469 , merged in commit f2a3e7d . Issue doesn't need to be open anymore.