Docs: Troubleshoot "pre-receive hook failed" error in mirrored repos

What does this MR do?

Adds a troubleshooting section about pre-receive hook failed errors in conjunction with You are not allowed to push code to protected branches on this project as reported internally.

Alternative wording I also considered:

find the following in [the `gitaly/current` log](../../../administration/logs.md#gitaly-logs)

    {
      …
      "error": "You are not allowed to push code to protected branches on this project.",
      "level": "error",
      "method": "POST",
      "msg": "Internal API error",
      …
    }

with a matching timestamp

I couldn't get the error reproduced, so I didn't yet create a bug report. As several tickets were found about this error, I priotized sharing the solution that a customer reported.

Related issues

gitaly#2923 (closed) because this here is one such example of vague errors message.

Author's checklist (required)

• Follow the Documentation Guidelines and Style Guide.
• If you have Developer permissions or higher:
  • [-] Ensure that the product tier badge is added to doc's h1.
  • Apply the documentation label, plus:
    • The corresponding DevOps stage and group labels, if applicable.
    • development guidelines when changing docs under doc/development/*, CONTRIBUTING.md, or README.md.
    • development guidelines and Documentation guidelines when changing docs under development/documentation/*.
    • development guidelines and Description templates (.gitlab/*) when creating/updating issue and MR description templates.
  • Assign the designated Technical Writer.

Do not add the feature, frontend, backend, ~"bug", or database labels if you are only updating documentation. These labels will cause the MR to be added to code verification QA issues.

When applicable:

• [-] Update the permissions table.
• [-] Link docs to and from the higher-level index page, plus other related docs where helpful.
• [-] Add the product tier badge accordingly.
• [-] Add GitLab's version history note(s).
• [-] Add/update the feature flag section.

Review checklist

All reviewers can help ensure accuracy, clarity, completeness, and adherence to the Documentation Guidelines and Style Guide.

1. Primary Reviewer

• Review by a code reviewer or other selected colleague to confirm accuracy, clarity, and completeness. This can be skipped for minor fixes without substantive content changes.

2. Technical Writer

• Technical writer review. If not requested for this MR, must be scheduled post-merge. To request for this MR, assign the writer listed for the applicable DevOps stage.
  • Ensure docs metadata are present and up-to-date.
  • Ensure Technical Writing and documentation are added.
  • Add the corresponding docs:: scoped label.
  • Add twdoing when starting work on the MR.
  • Add twfinished if Technical Writing team work on the MR is complete but it remains open.

3. Maintainer

1. Review by assigned maintainer, who can always request/require the above reviews. Maintainer's review can occur before or after a technical writer review.
2. Ensure a release milestone is set.
3. If there has not been a technical writer review, create an issue for one using the Doc Review template.