Release-docs pages should link to versioned online RefGuide

The official document page for Solr is missing the link to the live Solr Reference Guide. Only the link to PDF is there. In fact, one has to go to the WIKI, it seems to find the link.

It is also not linked from the release-specific documentation page either.

This means the search engines do not easily discover the new content and it does not show up in searches for when people look for information. It also means people may hesitate to look at it, if they have to download the whole PDF first.