favicon isn't renamed

if you have `"favicon": "my-image.png"` in your oranda config, it will get copied in as `my-image.png`, but the actual oranda site will still refer to `favicon.ico`. One of those two names needs to be changed (or I guess an error needs to be produced?).

fwiw in the case i tested, renaming the png to ico worked fine!

Reproduce the issue with an Oranda config containing "favicon": "my-image.png" and inspect the generated site and copied asset names. Trace where the configured favicon is copied and where the site refers to favicon.ico; done means the generated asset and reference use a consistent name, or the configuration produces a clear error.

Written by the indexing model from the issue text.