Docs: Abstract documentation generation into a new file

Abstract the introduction of documentation generation into
a new file.

Signed-off-by: Summer Qin <summer.qin@arm.com>
Change-Id: I0a47e21b426dbb53e8165d08c81e9174958cf6d0
diff --git a/docs/technical_references/instructions/index.rst b/docs/technical_references/instructions/index.rst
index 19a6e43..e9c95a0 100644
--- a/docs/technical_references/instructions/index.rst
+++ b/docs/technical_references/instructions/index.rst
@@ -5,7 +5,10 @@
     :maxdepth: 1
     :glob:
 
-    *
+    tfm_build_instruction
+    tfm_build_instruction_iar
+    tfm_run_instruction
+    documentation_generation
 
 --------------