Proposal: Add automated tests

Since this will be a big part of our documentation, we must have tests in here.

Here's how we could make them:
- on each PR
- use https://www.cypress.io/ to load corresponding demos on CodeSandbox (https://codesandbox.io/s/github/algolia/doc-onboarding/tree/SHA/demos/instantsearchjs/media) SHA being the latest commit SHA in the PR (example: ed25263)
- ensure that the page loaded and has a particular text in it (like the first product hit). If it has, then the demo works

Might be a little bit tricky to handle since the actual hit widget is commented.

Thoughts?

Start with the CodeSandbox demos under demos/instantsearchjs/media and the proposed Cypress flow for loading them at a PR's latest commit SHA. Done means each PR checks that the page loads and contains expected text, while accounting for the commented hit widget.

Written by the indexing model from the issue text.