Doc collection: keyword filter

## Overview
Nick's ready to tackle keyword search on `Official document collections`, and since we did some wide exploration for filtering a few months ago, we already have a head start on that effort. Let's get the specs and criteria ready to hand off to dev by the end of this sprint.

Keyword filtering on an official document collection will:
-

## Components
- Label for the field group
- EN: ""
- ES: ""
- Text input field
- Any states: focus/active, error

## Criteria
Abstract branch:
Dev criteria collection:

No files, tests, or implementation entry points are named. Start by reviewing the prior filtering exploration referenced in the issue and resolve the missing keyword-filter behavior, labels, states, and acceptance criteria. Done means a complete specification is ready to hand off to development.

Written by the indexing model from the issue text.