docs: `mdformat` description

[fyliu]

Overview

We need a description for the mdformat tool used for format markdown files: what it does, how it compares to markdown lint, how to setup and use it with an example.

Action Items

• create mdformat.md page in tools/
• write description
  • what it does
  • how it compares to markdown lint
  • how to setup and use it
• add example to format a file
• add example to run it in pre-commit

Resources/Instructions

• mdformat repo
• mdformat-mkdocs repo
• our pre-commit page - may need to fix link