Add license policies tutorial in documentation

[tdruez]

A suggested by @Atharex in #335 (comment), that feature is not well presented and hidden deep in the settings.

Paragraphs ideas for the tutorial:

1. General explanation about license policies and compliance alert
2. How to create a policies file, details about the format
3. Define the location of the policies file in the settings
4. How the compliance alert ok/warning/error/missing works

ok = self.Compliance.OK
error = self.Compliance.ERROR
warning = self.Compliance.WARNING
missing = self.Compliance.MISSING

 if error in alerts:
     return error
 elif warning in alerts:
     return warning
 elif missing in alerts:
     return missing
 return ok

5. Run a pipeline and look at the policies results

[ddmesh]

Sometimes there are files that are licensed under several licenses. So the user may choose the needed licences.
If such a case was detected I find those licenses listed in the table. in case I have gpl license and also a lgpl
license for one file, the compliance alert will show error.
Question: is there a way to set such a case to "warning" instead of "error" because a GPL was found.

The policies.yml file that I currently have, specifies GPL as error and LGPL as Warning.
Is there a way to let compliance alert being "Warning" if a file has a conflicting/multiple choice license?

Idea: specify a conditional logic within policies.yml