feat: add global functions to mdbook, allow documenting arguments and return values

[makspll]

ladfile builder will now parse docstrings of the following kind:

/// Arguments:
/// * `arg_name1` : doc1
/// Returns:
/// * `return_name` : doc2

And attaches those to newly added documentation fields on the arguments of functions and return types.

Also refactors mdbook preprocessor slightly, to make use of these, as well as re-structures the generated markdown.