Docs: deprecate old "Releases" doc

Problem to solve

• We have this old doc, pretty much outdated: https://docs.gitlab.com/ee/workflow/releases.html
• It links to the new Releases feature documentation: https://docs.gitlab.com/ee/user/project/releases/index.html

The old doc doesn't look good, maybe we could add that info to the new doc, deprecate the old document by deleting its content and setting up a redirect from the old to the new?

Who can address the issue

Docs team + ~"devops::release" eng.

Other links/references

I've noticed this when pinged in #26001 (closed)