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/technical_references/instructions/documentation_generation.rst b/docs/technical_references/instructions/documentation_generation.rst
index e6b4fc5..72e7e55 100644
--- a/docs/technical_references/instructions/documentation_generation.rst
+++ b/docs/technical_references/instructions/documentation_generation.rst
@@ -7,10 +7,8 @@
 - Reference Manual. Doxygen based.
 - User Guide. Sphinx based.
 
-Both documents can be generated in HTML and PDF format.
-
-Support for document generation in the build environment is not mandatory.
-Missing document generation tools will not block building the TF-M firmware.
+Both documents can be generated in HTML and PDF format and independently from
+building the project binary artifacts.
 
 ***************************
 Build TF-M Reference Manual
@@ -48,8 +46,8 @@
             .. code-block:: bash
 
                 cd <TF-M base folder>
-                cmake -S . -B cmake_doc -DTFM_PLATFORM=arm/mps2/an521
-                cmake --build cmake_doc -- tfm_docs_refman_html tfm_docs_refman_pdf
+                cmake -S docs -B build_docs
+                cmake --build build_docs -- tfm_docs_refman_html tfm_docs_refman_pdf
 
             The documentation files will be available under the directory ``cmake_doc/docs/reference_manual``.
 
@@ -104,10 +102,10 @@
         .. code-block:: bash
 
             cd <TF-M base folder>
-            cmake -S . -B cmake_doc -DTFM_PLATFORM=arm/mps2/an521
-            cmake --build cmake_doc -- tfm_docs_refman_html tfm_docs_refman_pdf
+            cmake -S docs -B build_docs -G"Unix Makefiles"
+            cmake --build build_docs -- tfm_docs_refman_html tfm_docs_refman_pdf
 
-        The documentation files will be available under the directory ``cmake_doc\docs\reference_manual``.
+        The documentation files will be available under the directory ``build_docs\docs\reference_manual``.
 
 *********************
 Build TF-M User Guide
@@ -154,10 +152,10 @@
             .. code-block:: bash
 
                 cd <TF-M base folder>
-                cmake -S . -B cmake_doc -DTFM_PLATFORM=arm/mps2/an521
-                cmake --build cmake_doc -- tfm_docs_userguide_html tfm_docs_userguide_pdf
+                cmake -S docs -B build_docs
+                cmake --build build_docs -- tfm_docs_userguide_html tfm_docs_userguide_pdf
 
-            The documentation files will be available under the directory ``cmake_doc/docs/user_guide``.
+            The documentation files will be available under the directory ``build_docs/docs/user_guide``.
 
             - Manually using the appropriate tools (`sphinx-build`_/ `Doxygen`_)
 
@@ -222,10 +220,10 @@
         .. code-block:: bash
 
             cd <TF-M base folder>
-            cmake -S . -B cmake_doc -DTFM_PLATFORM=arm/mps2/an521
-            cmake --build cmake_doc -- tfm_docs_userguide_html tfm_docs_userguide_pdf
+            cmake -S docs -B build_docs -G"Unix Makefiles"
+            cmake --build build_docs -- tfm_docs_userguide_html tfm_docs_userguide_pdf
 
-        The documentation files will be available under the directory ``cmake_doc\docs\user_guide``.
+        The documentation files will be available under the directory ``build_docs\docs\user_guide``.
 
 .. _sphinx-build: https://www.sphinx-doc.org/en/master/man/sphinx-build.html
 .. _Doxygen: https://www.doxygen.nl