docs(context-mgmt): add documentation for context management library

This patch adds some documentation for the context management library.
It mainly covers the design at a higher level, with more focus on
the cold boot and warm boot entries as well as the operations
involved during context switch. Further it also includes a section
on feature enablement for individual world contexts.

Change-Id: I77005730f4df7f183f56a2c6dd04f6362e813c07
Signed-off-by: Jayanth Dodderi Chidanand <jayanthdodderi.chidanand@arm.com>
diff --git a/docs/components/index.rst b/docs/components/index.rst
index 18522f8..3697026 100644
--- a/docs/components/index.rst
+++ b/docs/components/index.rst
@@ -28,3 +28,4 @@
    granule-protection-tables-design
    ven-el3-service
    ven-el3-debugfs
+   context-management-library