migrate from `markdownlint` to `remark-lint`

1. [`remark-lint`](https://github.com/remarkjs/remark-lint) is much more pluggable than `markdownlint`.
2. It can be integrated with `eslint` via [`esling-plugin-mdx`](https://github.com/mdx-js/eslint-mdx#mdxremark) (Don't worry, markdown is also supported).
3. It will never conflict with `prettier` via [`remark-preset-prettier`](https://github.com/JounQin/remark-preset-prettier)