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 has a lot to do with DI config, which is still a moving topic. I've documented here to illustrate what's possible with the DI config, but we might not want to document it straight away? (because it's not yet stable) I don't know, it's up to discussion. In any case, a guide dedicated to DI config should be added in the future.

If you want to see a rendered version of the guide: https://github.com/piwik/developer-documentation/blob/f2606256f41ddd85b350d8c4ad2cf93b2c548bf1/docs/logging.md

**Do not merge this for now, we need to wait till we enter the 2.11 release cycle**