Add `fediverse:creator` markup

@andypiper is already working on this.

Since Aug 14, 2026.

When links from matrix.org are shared on the Fediverse[^1], we have an opportunity for them to include a more prominent link to the Foundation's Mastodon profile.

We can [add `fediverse:creator` tags](https://docs.joinmastodon.org/user/profile/#attribution) to the site to point to `@matrix@mastodon.matrix.org` in the bottom of link cards shared on Mastodon[^2]

Bonus / for extra credit, this could be extended to individual blog author identities if desired, but would mean that we'd need a lookup table for blog author fedi handles somewhere. I think that's a nice-to-have and relatively low-overhead, so I'd implement general pages with the Foundation handle as linked creator, and blog posts with individual author handles, except where none exists -> fallback to Foundation handle[^3].

[^1]: specifically, Mastodon; I'm not clear whether any other ActivityPub platforms have added support for `fediverse:creator`
[^2]: needs an equivalent setting "Websites allowed to credit you" on the Mastodon account profile, to validate that matrix.org links should be allowlisted for showing `fediverse:creator` information.
[^3]: additional wrinkle because of course there is one, the Mastodon implementation only supports a single handle being credited, so for multi-author blog posts we would have to fall back to the first named author / or the Foundation.