[TF-M] Announcement: documentation build is separated from the main.

Hello,

From now on, documentation has no interference with the main build and runs as an independent CMake project. This reduces CMake configuration phase and not requires doc tools for the normal development anymore.

There are no changes for building the binaries, but documentation build shall start in the docs folder cmake -S docs -B build_docs cmake --build build_docs

More details are here: https://tf-m-user-guide.trustedfirmware.org/docs/technical_references/instru...

Thanks, Anton