Documentation

[loechel]

Currently the documentation contains copied and doubled text as well as German documentation parts.

That should be consolidated and translated / removed.

We should have a consistent documentation, so that user and developers could understand what is happening in RestrictedPython.

There should also be a documentation listing all the built-in names RestrictedPython relies on, e. g. _getattr_.

[icemac]

Partly fixed in #162.