Use `gh-pages` branch instead of `server-content`

While we do not use GitHub¹ to host our website, using `gh-pages`
as the branch that holds the content intended for our server² has
the added benefit that it removes any confusion regarding to what
the branch is used for.

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¹ https://pages.github.com/
² https://github.com/h5bp/html5boilerplate.com/wiki