Update Alpha to Experiment in URL + create redirect

What does this MR do?

Updating URL of https://docs.gitlab.com/ee/policy/alpha-beta-support.html to match the contents of the page, then creating a redirect to the new page. This is a Sid and David request.

Follow-up to this MR !116840 (merged)

Update 2 files

• /doc/policy/experiment-beta-ga-support.md
• /doc/policy/alpha-beta-support.md

Related issues

Moving docs to a new location?

Read the redirect guidelines first.

• 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.
• Update the link in features.yml (if applicable).
• Assign one of the technical writers for review.