workflow: run keywords check weekly (#21679)

[ti-chi-bot]

This is an automated cherry-pick of #21679

What is changed, added or deleted? (Required)

This PR changes the keywords check from a per-PR and push CI gate to a weekly, manually runnable, and repository-dispatch workflow.

After this change, the workflow:

• checks keywords.md on the configured branches, currently master and release-8.5;
• compares each docs branch with the TiDB parser branch of the same name;
• creates an issue only when keyword mismatches are found;
• skips parser download failures with a warning instead of reporting them as keyword mismatches;
• normalizes check-keywords.py to return 0 for success and 1 for any number of mismatches.

This reduces unnecessary disruption to unrelated PRs. Previously, every PR could be blocked by a keywords mismatch even when the PR did not touch keywords.md or parser-related content (see pingcap/docs#22923 (comment)). With the new behavior, keyword drift is still monitored regularly, but it is reported as a maintenance issue instead of interrupting unrelated documentation changes.

Which TiDB version(s) do your changes apply to? (Required)

• master (the latest development version)
• v9.0 (TiDB 9.0 versions)
• v8.5 (TiDB 8.5 versions)
• v8.1 (TiDB 8.1 versions)
• v7.5 (TiDB 7.5 versions)
• v7.1 (TiDB 7.1 versions)
• v6.5 (TiDB 6.5 versions)
• v6.1 (TiDB 6.1 versions)

What is the related PR or file link(s)?

• This PR is translated from: workflow: run keywords check weekly docs#22951
• Other reference link(s):

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

[qiancai]

/approve

[ti-chi-bot]

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