Set up filtering by site section

Done when

Users can filter results on the result page by site section, like the current site.

Implementation steps

Like our existing Google search, we want to run a filtered search based on the values that the user has selected in the checkboxes. We can most likely use all the same UI functionality (like how it runs a new query when the boxes are checked/unchecked).

Seems like we'll want to use this: https://www.elastic.co/guide/en/elasticsearch/reference/current/query-filter-context.html#filter-context

We know the section a page is in based on the gitlab_docs_section meta tag, which is included in our index with each page (https://gitlab-docs-test.kb.us-central1.gcp.cloud.es.io:9243/app/enterprise_search/content/search_indices/search-gitlab-docs-hugo/documents).