WIP New docs site

[laosb]

To build new docs, you'll need to have Node.js and Yarn installed on your system, and run:

# Install dependencies
yarn
# Run `build` in package.json
yarn build

in docs-src/.

Docs will be built into docs, where GitHub Pages treated as site root. So be sure to commit docs changes.

CI/CD is not included in this PR, so manual build is required after every docs update. yarn dev runs a local hot-reload-enabled docs server for easier debugging.

In the long run, using some 3rd-party static hosting like Netlify would be easier to release docs.

Docs engine is based on VuePress.

[MouriNaruto]

Thanks.