docs(hftest): add Shrinkwrap-based testing guide for Hafnium

This patch adds a new documentation page, `hafnium-tests.rst`, under
`docs/getting_started/`. It explains how to use Shrinkwrap to build and
run
Hafnium-based tests using `hftest.py`, as well as how to run the
standalone `hafnium-tftf.yaml` configuration.

The document covers:
- Shrinkwrap integration in `hftest.py` and test automation via
  `kokoro/` scripts
- Overlay structure (static vs dynamic)
- Manual Shrinkwrap setup for CLI workflows

This helps developers understand and use the Shrinkwrap-backed test
infrastructure for validating Hafnium SPMC configurations, FF-A flows,
and full stack integrations with TF-A and TFTF.

Change-Id: I3f8a177f69d518400efb3aa3a83bebecdc338b75
Signed-off-by: Jayanth Dodderi Chidanand <jayanthdodderi.chidanand@arm.com>
diff --git a/docs/getting_started/index.rst b/docs/getting_started/index.rst
index 7a29602..9c8a84c 100644
--- a/docs/getting_started/index.rst
+++ b/docs/getting_started/index.rst
@@ -11,6 +11,7 @@
    coding-style
    commit-style
    project-structure
+   hafnium-tests
 
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