Move spec/migrations/README.md to docs

What does this MR do?

Moves the database migration testing guide from a README.md file in our repository to our developer docs.

Moving docs to a new location?

Read the guidelines: https://docs.gitlab.com/ce/development/documentation/index.html#changing-document-location

• Make sure the old link is not removed and has its contents replaced with a link to the new location.
• Make sure internal links pointing to the document in question are not broken.
• Search and replace any links referring to old docs in GitLab Rails app, specifically under the app/views/ and ee/app/views (for GitLab EE) directories.
• [-] Make sure to add redirect_from to the new document if there are any Disqus comments on the old document thread.
• [-] Update the link in features.yml (if applicable)
• [-] If working on CE and the ee-compat-check jobs fails, submit an MR to EE with the changes as well (https://docs.gitlab.com/ce/development/documentation/index.html#cherry-picking-from-ce-to-ee).
• Ping one of the technical writers for review.