feat(website): add DocSearch as recommended by docusaurus

[s-pace]

👋 team,

I am working on DocSearch. We have an integration for Docusaurus. We thought it is a shame you can not also used this search that you can find on reactjs for example.

This PR will add DocSearch to the documentation website. It will allow an user to have a learn-as-you-type experience by displaying results thanks to a dropdown in a live way.

Let me know if you need anything.

[s-pace]

Any help needed on this @thatch & @jreese? You should only need to hit the button 🚀