Update contribution guidelines wrt pre-commit hook

[Manciukic]

What

We should update the contributor guidelines to point external contributors to use the devtool instead of the current "local" pre-commit hook setup.

Why

The pre-commit hook is unmaintained, unused by the team, and currently doesn't work if the setup is different from the devctr (see #5180).

How

The equivalent operation in the devtool is ./tools/devtool fmt.

Other information

We should keep the git-secrets check, moving it to the devtool if it's not already there.