Documentation style guide: make links section clearer

What does this MR do?

Improves the guidance on links in the documentation.

• Before
• After

I sought out documentation to understand better what lead to !18634 (merged)

A single bullet point currently documents use of relative (../) links and the .md/.html change.

• Get it wrong and it breaks our FOSS build, which means the edges are sharper than expected.
• It's quite a dense set of requirements for one bullet.
• We state (eg: workflow) that anyone can contribute to docs, and it's a great way for folk to contribute, so I think doc docs should make this super easy.

There's also a somewhat unclear bullet:

• To link from CE to EE-only documentation, use the EE-only doc full URL.

• I'd like to, but I don't know how to improve it.
• I think I interpreted /ee as referring to this; it's not clear to me how to identify EE-only documentation.
• Is there any EE-only documentation?

Related issues

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 and that you merge the equivalent EE MR before the CE MR if both exist.
3. If there has not been a technical writer review, create an issue for one using the Doc Review template.