.github/workflows/docs-pr.yaml: Potential fix for code scanning alert no. 2: Workflow does not contain permissions

[mykaul]

Potential fix for https://github.com/scylladb/scylla-monitoring/security/code-scanning/2

To fix the problem, explicitly declare minimal GITHUB_TOKEN permissions for this workflow or for the specific job. Since the job only needs to read the repository to build and test docs, contents: read is sufficient. Adding a permissions block ensures the workflow doesn’t accidentally get broader write permissions inherited from repo/org defaults.

The best fix without changing functionality is to add a workflow-level permissions block right after the name (or before on:) so it applies to all jobs in this workflow. That block should set contents: read. No further permissions are needed for the existing steps (actions/checkout, actions/setup-python, and make commands that run locally). Concretely, in .github/workflows/docs-pr.yaml, insert:

permissions:
  contents: read

near the top of the file. No new methods, imports, or other definitions are required, because this is purely a configuration change within the workflow file.

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[annastuchlik]

@dgarcia360 Please have a look.