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/Kconfig.iosched b/block/Kconfig.iosched
index 7a6b2f2..b89310a 100644
--- a/block/Kconfig.iosched
+++ b/block/Kconfig.iosched
@@ -26,7 +26,7 @@
 	regardless of the device parameters and with any workload. It
 	also guarantees a low latency to interactive and soft
 	real-time applications.  Details in
-	Documentation/block/bfq-iosched.txt
+	Documentation/block/bfq-iosched.rst
 
 config BFQ_GROUP_IOSCHED
        bool "BFQ hierarchical scheduling support"