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

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by:
Once this PR has been reviewed and has the lgtm label, please assign lance6716 for approval. For more information see the Code Review Process.
Please ensure that each of them provides their approval before proceeding.

The full list of commands accepted by this bot can be found here.

Details

Needs approval from an approver in each of these files:

• OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment