[WIP] Documentation

Ping @bakura10 @basz

Working to improve documentation. I have set a few goals with this PR:
 1. Make clear what SlmQueue is at first sight
 2. Move all extended documentation to a separate docs/ folder to make it more readable
 3. Provide some code samples so it's directly clear how easy SlmQueue is to be used

The docs/ folder will have separated files for different topics. It's not definitive, but think about:
 1. Introduction (why a queue system, abstraction layer, module architecture: job + queue + worker + cli)
 2. Configuration (queue plugin manager, job plugin manager)
 3. Job creation + dependency injection for jobs
 4. Queue aware
 5. Queue interface
 6. Worker interface, controller action, cli
 7. Events