doc: Convert semihosting readme to rST

This converts the semihosting readme to rST. I have tried to make only
cosmetic changes, but I did fix up the first link (which was broken).

Signed-off-by: Sean Anderson <sean.anderson@seco.com>
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index 5b42579..3e52053 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -11,6 +11,7 @@
    netconsole
    partitions
    cmdline
+   semihosting
 
 Shell commands
 --------------