Add rule to have only local links in global nav

What does this MR do?

For clarity, this MR updates the rules for global nav use to only use links local to docs.gitlab.com within the docs site global nav.

The global nav's inability to indicate that linked items will take you to a site different from docs.gitlab.com could cause confusion, and in-page navigation and look-and-feel are affected. The global nav should be a nav that collectively (globally) describes all of the pages within our documentation site (which unfortunately isn't currently the case), and not concern itself with items not on our site.

If there are links that we need to send readers to, those links should be found within documentation pages, with appropriate context and external link indicators.

I appreciate any comments people may have, and I'm attempting to get keep this as an MVC.

Mentioning @gl-docsteam for review of this proposed process change for global nav links.

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.

For suggestions that you are confident don't need to be reviewed, change them locally and push a commit directly to save others from unneeded reviews. For example:

• Clear typos, like this is a typpo.
• Minor issues, like single quotes instead of double quotes, Oxford commas, and periods.

For more information, see our documentation on Merging a merge request.

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.