Add standard admonition for SIG talks

[ewels]

For example: https://nf-co.re/events/2024/sig_animalgenomics_inauguration

It'd be nice to have a dropdown admonition that shows by default for all talks tagged (do we have tags for events) as special-interest-group. Explaining that the talk is linked.

Better still if there could be a front-matter field for linking an event to a specific SIG. Then we could use that to show an admonition explaining and linking there. Could also use this to show a list of SIG-specific meetings on the SIG web pages (potentially via a component? #2463 )