Add TOC to CONTRIBUTE.md

Hello,

I will be happy when the Table of contents has been added to CONTRIBUTE.md. Its structure should be verified using pre-commit.

We can use
https://github.com/thlorenz/doctoc
Example with pre-commit:
https://github.com/apache/airflow/blob/0c1477f/.pre-commit-config.yaml#L130-L138

Best regards