Documentation: fpga: move fpga overview to driver-api

Start of moving Documentation/fpga/*.txt to driver-api, including:
 - Add new directory driver-api/fpga
 - Add new file driver-api/fpga/index.rst
 - Add driver-api/fpga to driver-api/index.rst
 - Move Documentation/fpga/overview.txt to driver-api/fpga/intro.rst
 - Formatting and rewrites so that intro.rst will build cleanly
   and form a good introduction to the rest of the docs to be added.

Signed-off-by: Alan Tull <atull@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>