Move Websites and Blogs documentation in Marketplace addons

The Websites and Blogs documentation is to be moved to the Marketplace addons section for 5.8.
To do:

• Rename page as Nuxeo Sites and Blogs (package name)
• Put an excerpt to the page (1st two sentences should be ok)
• Update generic pages that mention websites and blogs:
  • Remove Moderate and Comment rights from Document Management Concepts page (to be tested first)
  • Working with documents page
  • Nuxeo Platform modules overview screenshot
• Add a section about Moderate and Comment specific rights in package documentation
• Create ADMINDOC page : DM dependency, no specific installation steps.
  To be tested first.