Docs: install pre-commit via pipx

[fyliu]

Overview

We need to update the pre-commit instructions so that developers know it's required for developers working on the project. Also we want to suggest a better option (pipx) to installing it than globally, which is bad practice.

Action Items

• [ ] Create a new file at docs/howto/tools/pre-commit.md
• [ ] Move instructions from contributing.md and leave a link there to the new file
• [ ] Update the instructions to use pipx
• [ ] Be sure to say that pre-commit is a required part of development
• [ ] check and release #189 from icebox

Resources/Instructions

• 190 is an alternative to this, but this is easier to do.

• pipx docs

[fyliu]

Closing this since this issue is already implemented by PR #230.