Use Redux logo as favicon on GitBook docs

[jtmthf]

Added favicon and apple-touch-icon to docs as requested in #1735.

I added the files to the logos directory as that seemed the most logical place to put them. Upon the docs:build step, the image files are copied into the built GitBook.

[gaearon]

Thanks! I couldn’t find a way to bust the cache and change the URL to something like favicon.ico?v2 so it’ll only be visible to those who haven’t visited the docs yet.

[jtmthf]

Another more flexible option would be to fork the default theme. As of GitBook 3.0 that's now an option that can be specified in the book.json. The documentation is sparse, but I think I could figure it out. Only consequence is that the theme would be behind on commits from the upstream theme.

[gaearon]

I don’t think it’s worth it, tbh. Let’s keep it simple!