Add more diagrams to clarify some concepts

[Tropicao]

Some parts of the training involve a lot of text and oral explanations, which makes the concept a bit more difficult to get for trainees. This is especially true for the first part, during which we explain the basic Linux concepts. Those parts could benefit from more diagrams to properly illustrate the concepts.

There are two "immediate" candidates for this improvement:

• virtual memory, MMU, process mappings and relationship with physical memory
• scheduling, preemption, interactions between processes of different classes on a system

This effort can be of course extended to other parts of the training where it is relevant

[lucaceresoli]

Done the first part (memory)