Add development contribution guidelines to the docs

The development contribution guidelines in the minterpy docs are currently still empty. We already have enough material to start creating one:

• The CONTRIBUTING.md: this document (mainly contains the mechanics of contributing) will serve as the TL;DR version of the full dev. contribution guidelines.
• The slides presented by @hernan68 at the minterpy group meeting on 21.04.2022: this should provide additional detail about the development workflow, policies, and conventions.

I'll start preparing a draft for this and will iterate with @hernan68 later on before merging.