Simplify and streamline documentation

[pbayer]

• [x] transfer complementary docs to Companion
• [x] simplify usage.md
• [x] provide/rewrite introductory examples

[zhangliye]

The examples of SimPy is organized in the categories of features. I find it is easy for the new users to write hie/her own simulation quickly according to the corresponding example. Some of the examples of are organized as the categories,

Condition events

• Bank Renege
• Movie Renege

Interrupts

• Machine Shop

Resources: Container

• Gas Station Refueling

[pbayer]

Now I rewrote the introductory examples giving one example each for

1. event based and
2. process based simulation.

Those are serious examples and no toy examples anymore. More examples are on DiscreteEventsCompanion.