Add developer documentation to the guide

[mspanc]

We need to have a developer documentation that explain:

• [ ] what are all these controllers, dispatchers and loggers,
• [ ] what is the process architecture being used for pipelines,
• [ ] what kind of messages flow between different parts of the system,
• [ ] what is the meaning of fields in the the internal structures,
• [ ] how pulling works,
• [ ] how integration with native code is designed.

[mat-hek]

Some of these things are already documented, but as comments in the code, e.g. element's internal architecture is briefly described here.