[16.0][IMP] contract documentation

[victor-champonnois]

EDIT : Help welcome to make pre-commit pass. The precommit in the CI throws an error ModuleNotFoundError: No module named 'pkg_resources'

I've been using the contract module quite intensively in the past year, and I wanted to contribute by improving the documentation. I have only a user knowledge of this module, please don't hesitate to point out any mistake I made.

I also should say that I've used mostly Customer contracts. Maybe there are some specific features in Supplier contracts that I missed.

Here are the changes I've made :

• converted the readmes to markdown to make editing easier
• added contents to USAGE and CONFIGURATION. More specifically, I tried to add more precisions about the different recurrence configurations.
• updated one old screenshots.

I realized afterwards I should have made the markdown conversion in a separate commit. However I doubt there will be a lot of issues there since the rst was quite basic.

I've used mostly the v16 version. If these changes is approved and if there hasn't been major changes in v18, I will port to v18.