documentation

Pull request #454: [DEV-2281] New top-level menu entry ("Extensions") added.

Merge in WEB/documentation from feature/DEV-2281-6.4-D to release/6.4

* commit 'b57471d6dfac6837d688099d94e6fd3e36d808fd': (21 commits)
  [DEV-2281] Updated files in "extensions" folder
  [DEV-2281] Updated "Extensions" page
  [DEV-2281] Updated "Extensions" page and related pages
  [DEV-2281] Updated page numbering in "Items" section
  [DEV-2281] Saved links from braking due to structure change
  [DEV-2281] Updated "Extensions" page and section structure
  [DEV-2281] Updated "Extensions" page
  [DEV-2281] Updated "Extensions" page
  [DEV-2281] Updated page structure
  [DEV-2281] Updated page structure, etc.
  [DEV-2281] Updated "Extensions" page
  [DEV-2281] Updated menu order
  [DEV-2281] Updated "Extensions" page
  [DEV-2281] Updated "Extensions" page
  [DEV-2281] Updated "Extensions" page
  [DEV-2281] Updated "Extensions" page
  [DEV-2281] Updated "Extensions" page
  [DEV-2281] Updated "Loadable modules" paragraph
  [DEV-2281] Updated "User parameters", "system.run", "External checks"
  [DEV-2281] Updated page layout, "Overview", "User parameters", "system.run"
  ...