Consider ranking the documentation search results

I had this experience this morning with the search results. My goals was
to find the documentation on `on_delete`.

- I searched for `on_delete`:
https://docs.djangoproject.com/en/3.2/search/?q=on_delete

There were 10 results displayed on the first page, and 11 pages in total.
6 of those results were pre 2.x release notes.

The most promising result was result number 10: `Model instance
reference`. https://docs.djangoproject.com/en/3.2/ref/models/instances/

Model instance reference doesn't have anything for on_delete on it's page.
i.e. `Phrase not found` when searching

I think Django is helped by a good search system for their documentation.
Therefor, consider adding a ranking of sorts that ranks the documentation.

- release notes are helpful when they are recent
- pages that don't have the queried text on them is less helpful.

Start with the documentation search results for `on_delete` at the linked Django 3.2 search URL, then inspect the current search implementation and how release-note pages are indexed. Done should mean exact or relevant matches rank ahead of unrelated older release notes and pages that do not contain the query, with behavior covered by appropriate search tests.

Written by the indexing model from the issue text.