Docs: Deprecate 'build_docs' folder

With the prefix 'build', user may delete 'build_docs' accidentally.
Deprecate 'build_docs' folder and move the only files 'conf.py' to
'docs' folder.

Change-Id: I701e279861afdc9b0f6cfd10b343643acd77e51d
Signed-off-by: Shawn Shan <Shawn.Shan@arm.com>
diff --git a/docs/getting_started/tfm_build_instruction.rst b/docs/getting_started/tfm_build_instruction.rst
index 5576327..cdfcab5 100644
--- a/docs/getting_started/tfm_build_instruction.rst
+++ b/docs/getting_started/tfm_build_instruction.rst
@@ -440,7 +440,10 @@
 .. code-block:: bash
 
     # Build the documentation from build_docs directory
-    cd <TF-M base folder>/ build_docs/
+    cd <TF-M base folder>
+    mkdir build_docs
+    cp docs/conf.py build_docs/conf.py
+    cd build_docs
     sphinx-build ./ user_guide
 
     # Build the documentation from a custom location