Add navigation to the hierarchy page to docs

What does this MR do and why?

Note, this depends on !76720 (merged).

Also needed for the release post: gitlab-com/www-gitlab-com!94074 (merged).

We are adding a new page to the left navigation which displays hierarchy based on tier.

Feature issue: #340844 (closed)

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.