Add test_addtitions to the TOC tree

As follow-up of [1]

[1] https://review.opendev.org/c/openstack/telemetry-tempest-plugin/+/900543

Change-Id: Ic55c2f0f879e5b0785246f0b1f0c88ca9f3fa172
diff --git a/doc/source/index.rst b/doc/source/index.rst
index 657cd89..3fdb643 100644
--- a/doc/source/index.rst
+++ b/doc/source/index.rst
@@ -15,6 +15,7 @@
    readme
    installation
    contributing
+   test_additions
 
 Indices and tables
 ==================