tools/update-meson.py is too undiscoverable

[user202729]

Problem Description

As in the title. It leads to unnecessarily too many confusion ( #39159 #39046 (comment) )

Proposed Solution

I suggest the following

• the meson GitHub CI action automatically runs the script before building, then gives a failure if git diff returns nonzero diff (but proceed with the build and test anyway to save time)
• pre-commit hook?
• at the head of each meson.build file, say "this file is autogenerated" etc. (or only "mostly autogenerated"? I don't know)

Maybe @tobiasdiez is the best one to do this. (I'm not entirely sure how to use the script either (just run it?), or is it supposed to be fully automatic)