docs: Use a nixpkgs patch instead of copy/pasting nixpkgs functions (#1011)

This makes the code more maintainable, as it only depends on our small
patch, instead of a large number of internals of nixpkgs.

docs/either_recursive.patch

@@ -0,0 +1,49 @@
+commit e067eb1f6da7994150f10854e5fd635ca8bd0e92
+Author: traxys <quentin+dev@familleboyer.net>
+Date:   Sun Jan 14 17:54:55 2024 +0100
+
+    lib.types: Include the suboptions of both types for either
+    
+    This allows to correctly gather the sub options for types of the form
+    `either <type> (submodule {...})`.
+    
+    This requires adding two new types: `eitherRecursive` and
+    `oneOfRecursive` that avoid infinite recursions for types like
+    configuration types that are often of the form `oneOf [atom (listOf
+    configType)]`
+
+diff --git a/lib/types.nix b/lib/types.nix
+index cea63c598321..d26982db6cc0 100644
+--- a/lib/types.nix
++++ b/lib/types.nix
+@@ -906,10 +906,16 @@ rec {
+            then functor.type mt1 mt2
+            else null;
+       functor = (defaultFunctor name) // { wrapped = [ t1 t2 ]; };
++      getSubOptions = prefix: (t1.getSubOptions prefix) // (t2.getSubOptions prefix);
+       nestedTypes.left = t1;
+       nestedTypes.right = t2;
+     };
+ 
++    # Handle recursive leaf types, avoiding an infinite recursion
++    eitherRecursive = t1: t2: (either t1 t2) // {
++      getSubOptions = _: {};
++    };
++
+     # Any of the types in the given list
+     oneOf = ts:
+       let
+@@ -917,6 +923,13 @@ rec {
+         tail' = tail ts;
+       in foldl' either head' tail';
+ 
++    # Handle recursive leaf types, avoiding an infinite recursion
++    oneOfRecursive = ts:
++      let
++        head' = if ts == [] then throw "types.oneOfRecursive needs to get at least one type in its argument" else head ts;
++        tail' = tail ts;
++      in foldl' eitherRecursive head' tail';
++
+     # Either value of type `coercedType` or `finalType`, the former is
+     # converted to `finalType` using `coerceFunc`.
+     coercedTo = coercedType: coerceFunc: finalType: