MAINTAINERS: Tweak nRF BSIM filters

The exclusion filter for bsim tests is currently
too broad. There is "infrastructure" files inside
these folders (not just in the top level folder)
and users may add more.
So let's change the filter to exclude tests source,
cmake and kconfig files, but catch the rest.

Also the documentation for the tests and workflows
were missing.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>