documentation: fpga: move fpga-region.txt to driver-api

Move Documentation/fpga/fpga-region.txt to
driver-api/fpga/fpga-region.rst.  Including:
 - Add it to driver-api/fpga/index.rst
 - Formatting changes to build cleanly as ReST documentation
 - Some rewrites for better flow as a ReST doc such as moving
   API reference to the end of the doc
 - Rewrite API reference section to refer to kernel-doc
   documentation in fpga-region.c driver code

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