Generate and render Atom feed

Notes:

- Templates should be rendered by calling the `render_template` lifecycle since apps may apply transformations before rendering it.
- Cache
- Limit (default and/or configurable)

Ref:

* https://www.rfc-editor.org/rfc/rfc4287
* https://validator.w3.org/feed/docs/atom.html

Start by locating the feed-generation and template-rendering entry points in the Beacon codebase, then read the `render_template` lifecycle notes in this issue. Use RFC 4287 and the W3C Atom validator to define the output requirements. Done means a valid Atom feed with caching and a documented default or configurable item limit.

Written by the indexing model from the issue text.