🔎📘 Add "Search documentation" based on keywords to the website

[dwiyatci]

TypeScript Version: 2.7.0-dev.201xxxxx

Code: -

Expected behavior: "Search documentation" input bar should be added for making easy to search relevant sections of the documentation based on keywords, like in, e.g. https://webpack.js.org, https://angular.io, https://reactjs.org.

Actual behavior: "Search documentation" does not exist yet.

[orta]

Closing as a dupe of https://github.com/microsoft/TypeScript/issues/26956 (though this does have better emoji usage...)