Improve logging documentation

[amol-]

There is a lot of documentaiton available in https://docs.pylonsproject.org/projects/pylons-webframework/en/latest/logging.html that TG is missing but there is a lot more than can be added too:

For example, while debugging a performance issue, I wanted to get debug output of sqlalchemy and kallithea. If I'd just set the DEBUG level on the console level, I'd also get all debug output of the other groups, like beaker, routes, etc.

It should be documented how to debug a specific part of the system by enabling logging only for that.