Add templates for pretalx

[tjarbo]

What type of PR is this?

• Add a new dev container collection
• Update to an existing dev container collection
• Documentation/spec update
• Other containers.dev site update (UX, layout, etc)

Description

This PR adds dev containers templates for the open source project pretalx for both app and app-plugin development. I've already discussed with the maintainer about an official setup (under the "pretalx" organization), but for now it is supposed to be a community contribution only.

Collection checklist

If your PR contributes a new collection, please utilize this checklist:

• Collection name
• Maintainer name
• Maintainer contact link (i.e. link to a GitHub repo, email)
• Repository URL
• OCI Reference (must be the collection root, e.g. ghcr.io/<owner>/<repo> — one entry per repository, not one per Feature; do not include http:// or https://)
• I acknowledge that this collection provides new functionality, distinct from the existing collections part of this index.