Doc: Various changes to the platform related documentation.

-Removed "Cross-Platform and Platform-Specific Development" page.
 Our overviews are presumed to be cross-platform and the platform
 specific content are in the respective platform pages. The page adds
 no value to our doc architecture.

-Removed group "platform-specific" group. It is not necessary as
 the platform pages already contains a link to their specific issues.
 I dispersed some of the links so then they are accessible from
 the platform pages.

Change-Id: I1d7eef83b8d5f1dc5af7d4c61833875da636cc39
Reviewed-by: Martin Smith <martin.smith@digia.com>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>
Reviewed-by: Sze Howe Koh <szehowe.koh@gmail.com>