scripts/requirements: Split & document requirement.txt

Split up requirements.txt into several files so that CI tools can
utilize/reference the specific requirements-<FOO>.txt they may need
while keep things in sync with the development.  This is to reduce
both time and amount of work CI actions due to python package install.

Create the following groupings:

1. BASE - needed to build or create zephyr images
2. BUILD-TEST - need to run compile/build tests
3. DOC - need to build the docs
4. RUN-TEST - need for runtime testing
5. EXTRAS - optional or useful for development/developers workflow

Also tried to add a comment about what or why a given package is being
pulled in for.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>