pandoc drops .title classes when rendering to docbook, so these are
effectively just paragraphs anyway. without support for including them
in a table of contents the complexity of parsing them in
nixos-render-docs won't be warranted.
The current logic assumes that everything that isn't a derivation is a
store path, but it can also be something that's *coercible* to a store
path, like a flake input.
Unnecessary uses of `lib.toDerivation` result in errors in pure evaluation
mode when `builtins.storePath` is disabled.
Also document what a `package` is.
This type correctly merges multiple option types together while also
annotating them with file information. In a future commit this will be
used for `_module.freeformType`
