Add more details to "Handling compatibility breakages" page

[Meorge]

Closes #11459 .

This PR adds more detailed instructions to the "Handling compatibility breakages" page, explaining how to generate the "Validate extension JSON" text that goes in the GH-*.txt file.

It also emphasizes the "Hash changed" error and gives some guidance on things to check when trying to fix it.

This is probably already assumed, but just in case: it'd be very good to get GDExtension peoples' OK on this before merging. The steps I laid out here make logical sense to me and they worked when I tested them with a PR of mine, but I certainly don't want to end up misleading people if there are steps or caveats I'm not aware of.

[Calinou]

Looks good to me.

[mhilbrunner]

Thank you! Merged.

[Meorge]

Thanks so much! 😄

One small note: I noticed this received the cherrypick:4.5 label; however, it references the new file-based system for PRs' compatibility breakages (using 4.x-stable/GH-#####.txt files instead of a single 4.x-stable.expected file) being introduced with 4.6. As such, would it make sense for this to be cherrypicked back to 4.5's documentation? This is made even trickier by the fact that the example given on the page takes place during development of 4.3, when this new system wasn't there either... 😅

[mhilbrunner]

Good point, removed the label for now, thanks for pointing that out :)

[Meorge]

No problem! Most of the PR content is also applicable to before the new file-based system; would it make sense for me to submit a pre-4.6-specific version for the sole purpose of cherrypicking? Given that's not where main engine development is now, it may be unnecessary, but otherwise I'd be happy to do that!