Added a documentation article for logging

References piwik/piwik#6813

I've added an article to document:

- how to use the new PSR-3 logger
- ~~how plugins can override its configuration~~

The second part was removed because it has a lot to do with DI config, which is still a moving topic. We do not want to expose that API yet.

If you want to see the removed part: https://github.com/piwik/developer-documentation/blob/f2606256f41ddd85b350d8c4ad2cf93b2c548bf1/docs/logging.md

This PR depends on https://github.com/piwik/piwik/pull/6813
> according to the log level configured by the user

according to the log level configured by the user in their config.ini.php ?