Add canonical link element from /tutorial page to haskell-servant.readthedocs.io

[m-renaud]

When doing a search for [servant tutorial] the top result is https://haskell-servant.github.io/tutorial/, but the tutorial appears to have been moved to http://haskell-servant.readthedocs.io. My guess would be that the latter is what most people are looking for so it may make sense to add a canonical link so that search engines rank it higher. Just a thought :)