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 <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Documentation/fpga/overview.txt

@@ -1,23 +0,0 @@
-Linux kernel FPGA support
-
-Alan Tull 2017
-
-The main point of this project has been to separate the out the upper layers
-that know when to reprogram a FPGA from the lower layers that know how to
-reprogram a specific FPGA device.  The intention is to make this manufacturer
-agnostic, understanding that of course the FPGA images are very device specific
-themselves.
-
-The framework in the kernel includes:
-* low level FPGA manager drivers that know how to program a specific device
-* the fpga-mgr framework they are registered with
-* low level FPGA bridge drivers for hard/soft bridges which are intended to
-  be disable during FPGA programming
-* the fpga-bridge framework they are registered with
-* the fpga-region framework which associates and controls managers and bridges
-  as reconfigurable regions
-* the of-fpga-region support for reprogramming FPGAs when device tree overlays
-  are applied.
-
-I would encourage you the user to add code that creates FPGA regions rather
-that trying to control managers and bridges separately.