Implement redirect handling

Add handling for two more kinds of redirects:

• Redirects for pages with redirect_to in the frontmatter (these). Any page with redirect_to in the frontmatter should redirect to the specified new location.

  • MR: Add handling for page-level redirects (!152 - merged) • Sarah German • 17.3

• Redirect handler for archived versions. If you request an archived page, we need to redirect to the Archives site when possible (e.g, https://docs.gitlab.com/15.11/ee/ci redirects to https://archives.docs.gitlab.com/15.11/ee/ci)

  • We can potentially now use a domain-level pages redirect for this (see https://docs.gitlab.com/ee/user/project/pages/redirects.html#domain-level-redirects).
    • However we do this, note that it will likely need to be a part of the release process (#39 (closed))