docs: Correctly generate documentation for types either a (submodule ...) (#929)

This requires pulling quite a lot of code from nixpkgs in order to
make the `either` type work correctly, the effects can be seen for
example in the documentation of `ollama.promps` and `ollama.actions`

docs/mdbook/book.toml

@@ -0,0 +1,10 @@
+[book]
+authors = []
+language = "en"
+multilingual = false
+src = "."
+title = "nixvim docs"
+
+[output.html.fold]
+enable = true
+level = 0