Add config for yamllint; Apply yamllint to all YAML files.

[kirkrodrigues]

Description

yamllint is a package for linting YAML files with options like enforcing all strings must be quoted. This PR adds a yamllint config file and applies it to all YAML files in the repo.

To run yamllint:

python3 -m venv yamllint-venv
. yamllint-venv/bin/activate
pip3 install --upgrade yamllint
yamllint .

The only files it should complain about are the ones from yamllint-venv.

We will add a workflow to run it in a future PR.

[LinZhihao-723]

lgtm