Apply many fixes re formatting of the document:
- Remove unnecessary quotation marks,
- Use ``backticks`` for inline code formatting,
- Fix incorrect break of a long TEMPLATECONF=.. line,
- Remove unnecessary break of `wget ...` line,
- Add/remove some spaces,
- Use ``#`` for comments in shell commands,
- Make GPIO capitalization consistent.

Signed-off-by: Wojciech Zmuda <wojciech.zmuda@huawei.com>

* Hardware supported boards modified to reference Supported Images.
* docs/index.rst modified to include Supported Images topic in toctree.

Signed-off-by: Arul Kumaran <arulkumaran.devarajan@huawei.com>

* All topics revised to reflect All scenarios OS instead of All scenario
  OS to maintain consistency, fixes issue#70

Signed-off-by: Arul Kumaran <arulkumaran.devarajan@huawei.com>