Add note to top of DOC and archived CONFnn spaces about documentation versioning

I've already added (on 15 April) a note to the top of the DOC space, letting readers know that this is the documentation for the most recent version of Confluence.

Now we should also add a note to the top of all the archived documentation spaces (CONFnn) mentioning that the documentation is for previous versions of Confluence and pointing to the DOC space for the most up-to-date docs.