fix(watch): improve watch solution, ignore _meta.json out of doc root dir

[zllkjc]

Summary

before:

1. rspress will watch all file in doc root dir, but only handle some config file.
2. all the _meta.json in cwd are watched.

now

1. only watch _meta.json files in doc root dir.

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

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[Timeless0911]

🤙