Move docs/ to Doxygen Documentation

@peller This moves the ARON and Memory documentation from docs/ to the Doxygen documentation. Currently, they appear as top-level entries in the ArmarX documentation.

Preview:

ARON:

Memory:

Also:

• The page about the "Mapping" segment in the RobotStateMemory has been removed in accordance with @reister
• Moves the "RobotUnit" and "GamepadUnit" pages (spurious top-level entries) to be a subpage of RobotAPI overview