Setup remark-lint for the docs

[sobolevn]

Feature request

As we have discussed with @sarahdayan the future roadmap of remark-lint adoption is:

1. I will set up the initial remark-based config at https://github.com/vuejs/vuepress/tree/master/packages/docs (estimate: 6.08.2019)
2. I will have to figure out what style rules does this repo already have, so the future diffs would be minimal (+1-2 days)
3. I will fix the style violations and mark PR as ready to be reviewed
4. Review, fixes, happiness

Later we can add more awesome plugins that enforces some best-practices like:

1. Dead links checker: https://github.com/davidtheclark/remark-lint-no-dead-urls
2. List of all plugins is here: https://github.com/remarkjs/remark-lint#list-of-external-rules

There's also an auto-formatter available: https://github.com/remarkjs/remark-lint#using-remark-to-fix-your-markdown (not sure about it, since I have never used it)

What problem does this feature solve?

It simplifies the documentation writing/refactoring/reviewing process by reducing the cognitive complexity on the reader and the writer.

Are you willing to work on this yourself?

Yeap, but any help is appreciated. 🙂