Update CONTRIBUTING.md to simplify hook instructions

[dcoeurjo]

Removed instructions for running all hooks on all files.

PR Description

your description here

Checklist

• I have installed and run the pre-commit hooks (cf CONTRIBUTING.md).
• Unit-test of your feature with Catch.
• Doxygen documentation of the code completed (classes, methods, types, members...)
• Documentation module page added or updated.
• New entry in the ChangeLog.md added.
• No warning raised in Debug mode.
• All continuous integration tests pass (Github Actions)