towncrier: Add new template for changelog

We will use this pipeline to generate changelog automatically
for each MR.

+ the configuration file in the root (each project has own)

We could use pre-commit instead of Gitlab CI but then we do not know
the branch to compare with. Some users can develop on main in their
forks (not ideal but happens).

Signed-off-by: Martin Kojtal <martin.kojtal@arm.com>