Add breadcrumbs to search results

Including breadcrumbs with search results provides helpful context to visitors. This could be especially useful on the Docs site since we have similar-looking pages that appear in different sections (e.g, feature flags for admins, feature flags for developers).

We can include these for both google.com searches and our own searches by adding metadata to our page templates: https://developers.google.com/search/docs/appearance/structured-data/breadcrumb#microdata

This will take two steps:

• Add breadcrumb metadata to pages in the proper schema.org format (!4011 (merged))
• Once Google has picked this up, display the breadcrumbs alongside results in the dropdown and advanced search result page

See also: #734 (closed)