Docs: Enable Extras docs building independently

To build tf-m-extra docs you can follow tf-m convention now:

>cmake -S docs -B build_docs
>cmake --build build_docs -- tfm_docs_extras_html

Signed-off-by: Anton Komlev <anton.komlev@arm.com>
Change-Id: Ia8ea218c0a75656945597824770707d47aa5f488
2 files changed