Updates `update-excerpts` to support any top-level .md file (other than CHANGELOG.md), rather than just README.md. This is useful for supplemental content, such as migration guides linked from the main README file.
Also makes some small improvements to the error messaging:
- The list of incorrect files is now relative to, and restricted to, the package. This makes the error message simpler, and ensures that changed files in other packages don't get listed.
- Adds a link to the relevant wiki docs, since this has been a source of confusion for newer contributors.
When determining whether or not to fail with `--fail-on-change`, only
look at .md files. In some cases, running the necessary commands (e.g.,
`flutter pub get`) may change unrelated files, causing fales positive
failures. Only changed documentation files should be flagged.
Also log the specific files that were detected as changed, to aid in
debugging any future false positives.
Fixes https://github.com/flutter/flutter/issues/111592
Fixes https://github.com/flutter/flutter/issues/111590
