Add favicons

[VojtechStep]

This PR adds Trebor's simplified Agda logo as favicons, adapting to dark and light themes of the browser.

Fixes #872

[fredrik-bakke]

Absolutely marvelous! Thanks for taking the initiative to make this small but awesome change

[EgbertRijke]

Btw where is this favicon supposed to show? I tried to look for it, but I can't find it anywhere.

[VojtechStep]

It's the image used for the tab icon in browsers, and once search engines reindex the website (which takes some time to happen), it will also appear next to the search results on e.g. Google or DDG.

[EgbertRijke]

For me it doesn't work

[EgbertRijke]

I still get:

even after restarting my browser

[VojtechStep]

That, uh, shouldn't happen? Did you try refreshing the tab? I was under the impression that modern Safari tries a little harder not to invent their own things that go counter to the rest of the web ecosystem.

[EgbertRijke]

Yep, tried everything

[fredrik-bakke]

For reference, I tried viewing the site in Safari too, and I got the same behavior as Egbert.

[VojtechStep]

Smh