Proposal: Add automated tests

[vvo]

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?