Add mdformat to pre-commit

[fyliu]

Fixes #232

What changes did you make?

• Added mdformat to autoformat markdown documentation files
• Autoformatted ISSUE_TEMPLATE markdown files and manually checked that they're right

Why did you make the changes (we will use this info to test)?

• Having a formatter will make markdown file structure more standardized so that contributors can focus more on the contents
• I set it to reformat md files as much as possible to set a good baseline

How to test

Either add some markdown or break something existing. There's no point running pre-commit run --all-files since the pre-commit.io bot already ran it.