Archive 5.8 documentation

Since Nuxeo Platform 5.8 is no longer maintained, we need to archive the 5.8 branch so it's not indexed by Google any longer.
Depending on the solution we'll implement, there are some things to be careful about:

• in other spaces, we may use some excerpts from 5.8 docs (in Drive for instance) -> search for 5.8 in other branches and update content so it doesn't break everything. Depending on the cases, we can copy content-paste content from 5.8 to more recent versions, completely ignore the reference, move the images in more recent branches, etc. Also check in static branch.
• we will also maybe need to update the version-override properties of more recent pages of they point to 5.8 pages and we delete that branch.

If we decide to just provide a PDF and not have online docs anymore, the PDF exports done before the migration to Metalsmith are available on the intranet.