docs: block: convert to ReST

Rename the block documentation files to ReST, add an
index for them and adjust in order to produce a nice html
output via the Sphinx build system.

At its new index.rst, let's add a :orphan: while this is not linked to
the main index.rst file, in order to avoid build warnings.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
diff --git a/block/ioprio.c b/block/ioprio.c
index 2e0559f..77bcab1 100644
--- a/block/ioprio.c
+++ b/block/ioprio.c
@@ -17,7 +17,7 @@
  *
  * ioprio_set(PRIO_PROCESS, pid, prio);
  *
- * See also Documentation/block/ioprio.txt
+ * See also Documentation/block/ioprio.rst
  *
  */
 #include <linux/gfp.h>