Support custom favicons

Cockpit docs currently use the BlueOS icon in browser tabs. It would be nice if the theme made use of an overrideable set of favicon files in the static folder, pulling them in using something like [this](https://github.com/patrickelectric/patrickelectric.github.io/blob/18710004e3e83014cfce24074504facaebdd7dcf/themes/hyde/templates/index.html#L4-L6) (presumably prepended with the `base_url`).

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Inherited from bluerobotics/ardusub-zola#5

Start by locating the theme template that emits the current BlueOS favicon and compare it with the linked index.html example. Check how the static folder and base_url are handled, then make favicon files overrideable there and verify that generated pages reference them correctly.

Written by the indexing model from the issue text.