Add DocSearch support

[gregberge]

Copy the following snippet at the end of the html HEAD tag:

Copy the following snippet at the end of the html BODY tag: Add a search input in your page if you don't have any yet. Then update the inputSelector value to a CSS selector that targets your search input field. Optionally customize the look and feel by following our styling guide. Check your configuration in our GitHub repository. Please open a pull request if want to leverage your configuration.