[CI] [POC] Added code samples testing with Deptrac

[mnocon]

Target: master, 4.6

Deptrac is a code testing tool for architecture.

I've added a very simple ruleset - code samples should not use classes outside of the Ibexa\Contracts namespace.

This tool should act as a:

1. sanity check when adding code samples (as it's easy to miss the non-Contracts usage that can be replaced with their Contracts counterpart)
2. point for discussion with developers (is there a Contracts-compliant way of writing the code sample)

But we will never get rid of all the errors, as they would require architecture changes on the DXP side - so we will have to keep expanding the baseline with new code samples.

Commits:

• f4593c0 adds the tool configuration
• f73c60e fixes some low hanging fruits and generates the baseline

Generated baseline:
https://github.com/ibexa/documentation-developer/blob/add-deptrac/deptrac.baseline.yaml