Docs: Update build commands

Build commands for TF-M docs are simplified.
Dependency repos are no more needed to build docs.

Update build commands in Open CI to fit with the latest TF-M codebase.
Remove redundant env params related to dependencies.

Signed-off-by: Xinyu Zhang <xinyu.zhang@arm.com>
Change-Id: I04d4669de9d986d497d52eb95c8e0f7e8800e87a
Depends-On: I726dc17d9aa547b8faaf35564e21f25aa9c207a9
2 files changed