Missing documentation about testing and Continuous Integration in DRAGONS.

[b1quint]

Based on comments on the last SUSD C&C meeting, I believe that I should add some documentation about tests and the existing Continuous Integration tools for DRAGONS within the repository.

It is important to note that we have two CI tools: GitHub Actions (public) and Jenkins (internal). Documentation on Jenkins should be only available for NOIR's Lab / GEMINI staff since it might contain sensitive information.

The format of this documentation should be in Sphinx/RST since it is the default documentation framework used in DRAGONS.

[b1quint]

Added some documentation to doc/DRAGONS/testing.rst containing information about running and writing tests. We might want to clean it up a bit and change the current theme, so it looks less polluted.