Add an example for joining queries

> I'd like to see more user-facing docs about how to use this (probably an example), but I'll punt that to a new issue.

_Originally posted by @alice-i-cecile in https://github.com/bevyengine/bevy/pull/11535#pullrequestreview-1928803815_


This should be an ECS example that demonstrates how to join queries, and ideally motivates why one might want to do so. We should demonstrate both filtered and unfiltered joins, and explain that this is an "inner join" in database terms.