Manual for adding new tests

[JenySadadia]

We need a manual/guide describing instructions for adding new tests to the new KernelCI system.
This would save time spent on investigating the workflow by new developers while adding new tests.

• Add documentation
• Deploy it on kernelci.org/docs (Maybe a new page under https://kernelci.org/docs/api/?)

[pawiecz]

Part of this task will be to verify template rendering for added tests. Very quick overview of rendering process (using data from pipeline configuration) has been posted in kernelci/kernelci-project#358 (comment) together with an example of kci job generate tool usage in kernelci/kernelci-project#358 (comment).

This resulted in two PRs: kernelci/kernelci-core#2565 and #623 (which could potentially be used as reference later). The latter also includes author's verification process #623 (comment)