Update service deployment documentation

Updates the old portability model page to better describe how
service provider deployments for different environments and
platforms are organised. Introduces a more flexible directory
structure that improves reuse and extensibility for service
provider deployments.

Signed-off-by: Julian Hall <julian.hall@arm.com>
Change-Id: I5635c38210cbb55d993150e8efe9e84b95acdf49
diff --git a/docs/developer/index.rst b/docs/developer/index.rst
index 1fdccae..4ea55f5 100644
--- a/docs/developer/index.rst
+++ b/docs/developer/index.rst
@@ -7,7 +7,7 @@
 
     arch-overview
     project-structure
-    portability-model
+    service-deployment-model
     service-access-protocols
     service-locator
     software-requirements