Add .spi.yml for SPI-hosted API docs

[ahti]

Swift Package Index can generate DocC documentation and host it on the Cadova package page. Docs for that feature here.

This adds a .spi.yml file that enables documentation generation.

I considered adding a link to the docs to the readme in this pr as well, but I think its better to wait for that until docs are built (they say it might take 24hrs or so) and we had a look at the results. Once that looks good I can follow up with a pr to adjust the readme.

It might be good to add links in the wiki where relevant, too. Maybe in the future it would be good to transition the wiki pages to DocC articles or sth as well, so that they are versioned and can more easily link to API docs, but I'm not very experienced with DocC so I can't promise I'd be the one to do that.

[tomasf]

Oh cool, I didn't know this was a thing. Looking forward to seeing it in action. Thank you!

And yes, switching to DocC for conceptual docs does seem like a good idea. I went with the GitHub wiki because it seemed straightforward and accessible. Symbol docs are a no-brainer with immediate value, but documentation catalogs always seemed like more effort than they're worth. I suspect AI agents can make quick work of that now, though, and with SPI hosting it could definitely be a viable replacement.