Verify internal URL configuration.

What does this MR do?

Adding instruction to check that gitlab-shell installed on each Gitaly instance can access GitLab instance by using internal_api_url value from the configuration. If it is not configured or configuration is incorrect SSH-based cloning and pushing will fail for such Gitaly-s.

Example of incorrect configuration output:

Internal API available: FAILED - Internal API unreachable

Internal API available: FAILED - 401 Unauthorized

Successful verification:

Internal API available: OK
Redis available via internal API: OK

Related issues

Closes: gitaly#2503 (closed)

Author's checklist

• Follow the Documentation Guidelines and Style Guide.
• If applicable, update the permissions table.
• Link docs to and from the higher-level index page, plus other related docs where helpful.
• Apply the documentation label.

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

• Optional: 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.

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.