Require users to provide their own `:nuzzle/render-content` functions

[stelcodes]

Instead of creating a render-content function for the users, require them to create their own and put it under the :nuzzle/render-content key for every page in the config. The function should take a single argument, the page map. This means the :nuzzle/content key should be removed. This needs to come after #133 because this is impossible to do in an EDN file.

Nuzzle will expose a parse-md-file function that users can use to convert their Markdown functions into hiccup. This format also makes using Selmer a lot simpler. Instead of exposing a bunch of markdown processing options, now the user has complete control just be using functions. Nuzzle should trust the user is capable of using functions to define behavior!