Developers documentation

The current [documentation](https://kernelci.org/docs/api/) is still fairly incomplete and high-level. It conveys the purpose of the new API and pipeline and it presents basic usage for the simplest use cases, which is a good way to start getting the attention of potential users, but at this point it still doesn't contain any relevant information for contributors: API internals, models, howtos for developers, configuration, etc.

This suggests that parts of the design are still in flux and not fully stabilized.

Any person that wants to contribute needs to read and understand the code, which also lacks proper documentation in most places. This is risky because a contributor might draw certain design conclusions from the code that may not be right or that could be provisional.

Adding a stable documentation for the API parts that are well established should be a priority to encourage external contributions.

Start with the current API documentation at https://kernelci.org/docs/api/ and compare it with the API code to identify established areas suitable for contributor documentation. Document the stable API internals, models, developer howtos, and configuration; completion means contributors can understand and work with those areas without relying only on the code.

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