Add structure to add PyMC models to posteriordb

[MansMeg]

Models working with:
https://docs.google.com/spreadsheets/d/17sw7XJEN3Vbvt1681rPy1wy9KmNF9mcW2-bu8F2Wu5I/edit?gid=1218617168#gid=1218617168

Decisions:

• Rename folder to pymc
• Create a pymc-mini-guide in CONTRIBUTE (we only want the model object) - check the posteriordb overhaul (@JTorgander will do this as part of the test document)
• Add prophet as issue to add/add to google sheets above https://github.com/stan-dev/posteriordb/pull/311/files#diff-5ac6d5f6b7e234400ba74786c887c33a76dc4105090622075b700382b8842c59 (Add 40 complex PyMC model implementations #320)
• Create PR for tested model
• Check that info files has been updated
• Check that we only include model and update info - no additional files for all models
• Rename to _discrete posterior_database/models/pymc/M0_model.py and add marginal model also (Adding reference draws for discrete m0 posterior #313 )