docs: Decouple documentation build from the main

 - Documentation is a standalone CMake project in /docs
 - Exclude TFM_VERSION to a dedicated version.cmake file
 - Move /doxygen into /docs folder
 - Adjust documentation for building documentation :)
 - Suppress warnings of PythonModules mismatch in docs build

Signed-off-by: Anton Komlev <anton.komlev@arm.com>
Change-Id: I726dc17d9aa547b8faaf35564e21f25aa9c207a9
diff --git a/docs/tfm_env.py.in b/docs/tfm_env.py.in
index cc2f8bf..16292d2 100644
--- a/docs/tfm_env.py.in
+++ b/docs/tfm_env.py.in
@@ -9,7 +9,7 @@
 # by cmake and evaluated by the Python builder
 
 cmake_env = { "SPHINX_TMP_DOC_DIR"         : "@SPHINX_TMP_DOC_DIR@",
-              "TFM_ROOT_DIR"               : "@CMAKE_SOURCE_DIR@",
+              "TFM_ROOT_DIR"               : "@CMAKE_SOURCE_DIR@/..",
               "PLANTUML_JAR_PATH"          : "@PLANTUML_JAR_PATH@",
               "Java_JAVA_EXECUTABLE"       : "@Java_JAVA_EXECUTABLE@",
               "DOXYGEN_EXECUTABLE"         : "@DOXYGEN_EXECUTABLE@",