Unify naming and titles of platform support pages

  - The main platform support page for a platform is just foo.html
  - Sub pages are prefixed, e.g. foo-bar.html
  - All titles are Qt for Foo.
  - The unused file platform-notes.qdocinc was removed

Change-Id: Ie00b4c1dde1d3e80efbe6cfa5c191b3501ebde2b
Reviewed-by: Paul Wicking <paul.wicking@qt.io>
Reviewed-by: Venugopal Shivashankar <Venugopal.Shivashankar@qt.io>