Hugo's RSS template doesn't transitively include entries for all of a section's subpages

For reference, see:

- https://github.com/open-telemetry/opentelemetry.io/issues/4440

Related(?) / resources:

- https://github.com/gohugoio/hugo/blame/master/tpl/tplimpl/embedded/templates/_default/rss.xml
- https://github.com/cncf/dot-org-hugo-theme/pull/46