Lint docs

[alexeyr-ci]

Since we use Gatsby to build the website, and it uses Remark to process Markdown, https://github.com/remarkjs/remark-lint should check its validity.

It should include a custom rule to ban relative links inside docs, see https://github.com/shakacode/react_on_rails/blob/c2993d2550950bdbcfe012a2c034eadd6c8eb923/docs/contributor-info/pull-requests.md#links-to-other-docs.

Alternative (maybe for additional rules): https://github.com/DavidAnson/markdownlint