Add search functionality to the documentation section

The [documentation](https://linkerd.io/2.10/overview/) section of the website contains drop-down menus with links to various sections. It would be nice if there was a way of searching through the documentation without having to click on the drop-downs and manually looking for a specific section. Maybe a search bar that dynamically filters the results as the user enters text?

Start at the linked documentation section on linkerd.io and inspect how its drop-down navigation exposes documentation links. Determine how search should cover those sections and how results should be filtered as text is entered. Done means users can search the documentation without manually opening each drop-down.

Written by the indexing model from the issue text.