Rename mock navigation YAML file

What does this MR do and why?

Renames the mock navigation.yaml file we use for testing the global nav with RSpec.

Since it's intentionally very similar-looking to the real navigation.yaml file and has the same name, it's too easy to accidentally edit the wrong file. We can just rename the mock one to avoid confusion. 😄

How to set up and validate locally

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

1. Configure a local GitLab Docs environment: https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/doc/setup.md.
2. Verify rspec tests still pass: make rspec-tests

Merge request 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 merge request.