Provide search functionality

[kumarshantanu]

Google site-search provides search facility for keywords within a website. Consider supporting search-by-keyword feature.

[seancorfield]

I'm am resurrecting this site at https://clojure-doc.github.io so feel free to recreate this issue at https://github.com/clojure-doc/clojure-doc.github.io/issues or submit a PR https://github.com/clojure-doc/clojure-doc.github.io/pulls