Adds favicons to jekyll docs site

[JellyTitan]

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I think the Sofle V1 logo is charming, so I turned it into a favicon.

The favicon html is ingested as a jekyll include. Favicons and manifests are in separate directory.

The legacy Sofle logo logo3.svg had some cruft in the file from an older KiCad export that needed to be removed before it could be used for the favicon. I included the cleaned up file in the design folder as logo3b.svg.