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/CMakeLists.txt b/docs/CMakeLists.txt
index 85f63f0..950f49b 100644
--- a/docs/CMakeLists.txt
+++ b/docs/CMakeLists.txt
@@ -36,8 +36,8 @@
 )
 add_custom_command(OUTPUT ${SPHINX_TMP_DOC_DIR}/conf.py
     COMMAND ${CMAKE_COMMAND} -E make_directory ${SPHINX_TMP_DOC_DIR}
-    COMMAND ${CMAKE_COMMAND} -E copy ${CMAKE_SOURCE_DIR}/build_docs/conf.py ${SPHINX_TMP_DOC_DIR}/conf.py
-    MAIN_DEPENDENCY ${CMAKE_SOURCE_DIR}/build_docs/conf.py
+    COMMAND ${CMAKE_COMMAND} -E copy ${CMAKE_SOURCE_DIR}/docs/conf.py ${SPHINX_TMP_DOC_DIR}/conf.py
+    MAIN_DEPENDENCY ${CMAKE_SOURCE_DIR}/docs/conf.py
     BYPRODUCTS ${SPHINX_TMP_DOC_DIR}
 )