Add API-based autocomplete search

Add an in-page, API-based search solution (meaning the browser acts as the search client) to the default UI. This means adding a search box somewhere on the page, preferably above the navigation sidebar. When text is entered into the box, search results should be displayed in an overlay.

Done criteria:

• select an API-based search solution (algolia, swiftype, etc)
• read API key for search (if applicable) from site.keys
• add a search box to the page
• display search results in overlay