doc: Add initial Kronos documentation

Add initial documentation with:
* conf.py for the Sphinx Documentation Builder
* index.rst for the index
* license file documentation
* requirements.txt with dependencies for building the docs

Changelog: other
Signed-off-by: Tomás González <tomasagustin.gonzalezorlando@arm.com>
Signed-off-by: Peter Hoyes <Peter.Hoyes@arm.com>
Signed-off-by: Robbie Cao <robbie.cao@arm.com>
Signed-off-by: Filipe Rinaldi <filipe.rinaldi@arm.com>