docs/dsp: improve docstrings for effects, filters, library overview

[vk2seb]

Some additions to clean up holes in the documentation on existing DSP components:

• Add an overview of how the DSP library is structured and how different components are connected together.
• Audit dsp.effects, dsp.delay_effect, dsp.filters, dsp.mix and dsp.oscillators for missing docstrings and populate them.
• Add a test case for the OnePole filter, as it was missing
• Parameterize the coefficient type for MatrixMix.