Create Advanced Search Migrations styleguide

What does this MR do and why?

This MR makes a few documentation changes:

• Advanced Search migrations should have a styleguide so I moved the relevant information into it's own file. Updated all references
• Rename the Elasticsearch knowledge file to Advanced Search knowledge because GitLab supports Open Search and potentially will be adding Zoekt as another supported search engine (in addition to Elasticsearch)

Note: I'm also adding new navigation entries in the documentation which depend on these changes in gitlab-docs!3582 (merged)

Screenshots or screen recordings

N/A

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.