Explicitly model joint events

We currently do not explicitly model joint events. Instead, we kind of hint at it, by allowing multiple `` tags at the volume level. This creates some issues:

- We use the same mechanism as a kind of tag for workshops
- There is no event page for the associated event unless it is redundantly created (e.g., we have ijcnlp-2025, but not aacl-2025)

We should consider explicitly modeling this. I suggest a `` tag at the event level:

```xml

aacl
ijcnlp
2025
...
```

This would create the event page `/events/ijcnlp-aacl-2025`, and also redirect to it (via the `` tags) from `/events/ijcnlp-2025` and `/events/aacl-2025`.

_Originally posted by @mjpost in https://github.com/acl-org/acl-anthology/pull/7803#discussion_r2948535486_

Begin by tracing how volume-level venue tags currently produce event pages and redirects; the issue names no implementation files or tests. Define the event-level venue model for joint events, create the /events/ijcnlp-aacl-2025 page, and redirect the individual venue paths to it.

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