Move ADT plugin to the ADT help doc and combine the three setup guides
into one page that is dynamic based on a URL parameter.
Change-Id: If8a8855bdb8e8ec9b0eefedd35ae24ea60497741
The SDK Manager page is needed particularly as a reference by
some pages rather than pointing to the installation guide.
Change-Id: Ie835e310f62cdc88bd6318e82f48e36bdd8e117a