[docs] Implement site search

[ghost]

See lit/lit#752.

From that issue's description:

"Short term proposal: redirect to google.com/ with site:lit-html.polymer-project.org like we do on polymer-library.polymer-project.org
Longer term proposal: investigate docsearch, new integration with custom search, or something else."

[moderndeveloperllc]

@katejeffreys Looks like #464 fixed this.

[ghost]

@moderndeveloperllc thanks!