docs: Use direct URL for TF-M requirements.txt

The "Prerequisites" document contains instructions for installing build
dependencies, *before* the user builds the SDK. To avoid breaking this
workflow and causing confusion, we should not rely on the TF-M that is
fetched by a CMake build. Thus we replace the path to requirements.txt
from a CMake build path to a direct URL.

A downside is this adds extra maintenance overhead for keeping the URL
up to date with the TF-M version in the SDK. However, documentations
should be easy to follow. Keeping versions up to date is also something
we have to do for various other things in the docs such as toolchain
versions.

Signed-off-by: Lingkai Dong <lingkai.dong@arm.com>