Add schema.org structured data to events page

[jessealama]

Adds JSON-LD structured data to the events page for better search engine indexing using schema.org/Event markup. Fixes #738

Notes and potential areas for feedback

• data/events.yaml now allows city, country, and venue fields, all optional.
• The preferred way to indicate countries in Schema.org is to use the ISO 2-letter uppercase alpha format. As it stands now, location is freeform text. For the country field we have no checks, so it still remains freeform, but one might imagine defining a check. Another approach would be to render e.g. "DE" as "Germany".
• I ran into rate limiting problems when developing this locally, and I was able to get around this by setting the NODOWNLOAD environment variable. I'm not sure if this is quite right; any feedback welcome.

[jessealama]

Example of the generated markup for an event:

<li> <a href="https://pitmonticone.github.io/ItaLean2025/">ItaLean 2025: Bridging Formal Mathematics and AI</a> (Bologna, IT. December 9–12, 2025)
    <small class="align-middle">
        <span class="badge badge-secondary align-middle event-tag-conference">conference</span>
    </small>
    
    <script type="application/ld+json">
{
  "@context": "https://schema.org",
  "@type": "Event",
  "name": "ItaLean 2025: Bridging Formal Mathematics and AI",
  "url": "https://pitmonticone.github.io/ItaLean2025/",
  "startDate": "2025-12-09",
  "endDate": "2025-12-12",
  "eventAttendanceMode": "https://schema.org/OfflineEventAttendanceMode",
  "location": {
    "@type": "Place",
    "name": "Bologna",
    "address": {
      "@type": "PostalAddress",
      "addressLocality": "Bologna",
      "addressCountry": "IT"
    }
  },
  "eventStatus": "https://schema.org/EventScheduled"
}
    </script>
    
</li>

[bryangingechen]

Indeed, the NODOWNLOAD env var is mentioned in our README; the other option is to pass a GitHub token.

I hadn't heard of this before but it looks like it's relatively widely used on other such community pages, cf. rust foundation events, python events, so I'm not opposed to this. I am a little wary of introducing a node dependency, even if only in CI. Is there a comparable Python package for validation that we could run as part of make_site.py?

[jessealama]

Indeed, the NODOWNLOAD env var is mentioned in our README; the other option is to pass a GitHub token.

I hadn't heard of this before but it looks like it's relatively widely used on other such community pages, cf. rust foundation events, python events, so I'm not opposed to this. I am a little wary of introducing a node dependency, even if only in CI. Is there a comparable Python package for validation that we could run as part of make_site.py?

Thanks for taking a look! I reailze this is probably a bit of a curveball PR. I'll take a look at equivalent Python tools to see what can be done there; surely there's an equivalent.

[jessealama]

Converting to draft while I investigate Python equivalents for JavaScript's structured data test tool.

[jessealama]

I found pydantic2-schemaorg. It's a neat package (and could also be used elsewhere, too) that does just what we want. I've removed the NPM/JS setup.

[jessealama]

ping @bryangingechen