Docs: Add some details to the User Defined Redirects

[akien-mga]

• Move the "Implementation" section further up as a note, so that it's
  more visible (I missed it myself and spent some time trying to test
  redirects on 200 which do not work).
• Add mentions to the use case of setting Exact Redirects to have
  Page Redirects but applied only to a specific version. It's common
  to move pages only in the latest branch/future stable branches,
  without doing the change in older branches.
• The button to confirm a redirect is now named "Add".

---

I'm happy to amend the commit as needed if you disagree with some of the proposed changes.

[stsewd]

Thanks for the update!

[stsewd]

this is a little out of date, we run some logic when serving docs now. I think we can just mention that redirects only work on 404 (and maybe leave the second paragraph that mentions the js solution too).

readthedocs.org/readthedocs/proxito/views/serve.py

Lines 126 to 135 in c857c6c

	# TODO: un-comment when ready to perform redirect here
	# redirect_path, http_status = self.get_redirect(
	# final_project,
	# lang_slug,
	# version_slug,
	# filename,
	# request.path,
	# )
	# if redirect_path and http_status:
	# return self.get_redirect_response(request, redirect_path, http_status)

I think at this point we only need to make the decision on how to implement them and how to make it in a way that doesn't introduce a lot of queries to the db.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[akien-mga]

Sorry for the late update. I've addressed the above feedback (the rewrite of the .. note:: likely needs another proofreading).

[stsewd]

Thanks!