doc: Move content from Porting Guide

The Porting Guide document is broken down into separate,
per-topic documents that all live under the new "Porting"
chapter.

Change-Id: I269fa06cbf6544ce3c76700d1fcf3a669793c483
Signed-off-by: Paul Beesley <paul.beesley@arm.com>
Signed-off-by: Jimmy Brisson <jimmy.brisson@arm.com>
diff --git a/docs/index.rst b/docs/index.rst
index 1436767..e8eee2f 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -9,9 +9,9 @@
    about/index
    getting_started/index
    process/index
-   porting-guide
-   implementing-tests
    design
+   implementing-tests
+   porting/index
    change-log
    license