DOC: formulate main 2-pseudoscalar intensity with SymPy

[redeboer]

This PR adds the first notebook that attempts to formulate the intensity for a two-pseudoscalar system symbolically with SymPy:

For now, only the main intensity sum is formulated. Definitions or values for the amplitudes $[l]^{(\pm)}_{m,k}$ will be specified in follow-up PRs in the form of a dict mapping (like how it is done in AmpForm or in the polarimetry study).

Some additional goodies:

• Added infrastructure so that the notebooks are automatically rendered as HTML pages with Jupyter Book.
• Added a developer set-up for Python so that the environment can be recreated with conda env create and all checks can be run locally and through CI with pre-commit.
• Added some VSCode settings and recommendations, so that the dev set-up works out-of-the-box.

Instructions for the above are found in under README.md.