Docs: Alleviate securecookie error in Pages

What does this MR do?

Summarizes some advice from @cat and @rvzon against a failure to enable access control for GitLab Pages. gitlab-pages/current shows:

{
  "error": "securecookie: error - caused by: securecookie: failed to generate random iv",
  "host": "…PAGES…DOMAIN…",
  "level": "error",
  "msg": "Failed to save the session",
  "path": "/",
  "state": "",
  "time": "…T…Z"
}
{
  "correlation_id": "",
  "duration_ms": 0,
  "host": "…PAGES…DOMAIN…",
  "level": "info",
  "method": "GET",
  "msg": "access",
  "pages_host": "…PAGES…DOMAIN…",
  …
  "status": 500,
  …
  "uri": "/",
  …
}

Related issues

Internal tickets: 143580, 141658, 175110

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.
  • If working on UI text, add the corresponding UI Text 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.

For more information about labels, see Technical Writing workflows - Labels.

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.