doc: reshape python-interpreter-table.nix into a normal callPackage

Before, it produced a string, not a derivation.

doc/doc-support/python-interpreter-table.nix

@@ -1,14 +1,15 @@
-# For debugging, run in this directory:
-#     nix eval --impure --raw --expr 'import ./python-interpreter-table.nix {}'
-{ pkgs ? (import ../.. { config = { }; overlays = []; }) }:
+# To run this derivation, `nix-build -A nixpkgs-manual.pythonInterpreterTable`
+{
+  lib,
+  writeText,
+  pkgs,
+  pythonInterpreters,
+}:
 let
-  lib = pkgs.lib;
-  inherit (lib.attrsets) attrNames filterAttrs;
-  inherit (lib.lists) elem filter map naturalSort reverseList;
-  inherit (lib.strings) concatStringsSep;
-
-  isPythonInterpreter = name:
-    /* NB: Package names that don't follow the regular expression:
+  isPythonInterpreter =
+    name:
+    /*
+      NB: Package names that don't follow the regular expression:
       - `python-cosmopolitan` is not part of `pkgs.pythonInterpreters`.
       - `_prebuilt` interpreters are used for bootstrapping internally.
       - `python3Minimal` contains python packages, left behind conservatively.
@@ -16,7 +17,8 @@ let
     */
     (lib.strings.match "(pypy|python)([[:digit:]]*)" name) != null;
 
-  interpreterName = pname:
+  interpreterName =
+    pname:
     let
       cuteName = {
         cpython = "CPython";
@@ -26,16 +28,16 @@ let
     in
     "${cuteName.${interpreter.implementation}} ${interpreter.pythonVersion}";
 
-  interpreters = reverseList (naturalSort (
-    filter isPythonInterpreter (attrNames pkgs.pythonInterpreters)
-  ));
+  interpreters = lib.reverseList (
+    lib.naturalSort (lib.filter isPythonInterpreter (lib.attrNames pythonInterpreters))
+  );
 
-  aliases = pname:
-    attrNames (
-      filterAttrs (name: value:
-        isPythonInterpreter name
-        && name != pname
-        && interpreterName name == interpreterName pname
+  aliases =
+    pname:
+    lib.attrNames (
+      lib.filterAttrs (
+        name: value:
+        isPythonInterpreter name && name != pname && interpreterName name == interpreterName pname
       ) pkgs
     );
 
@@ -45,18 +47,17 @@ let
     interpreter = interpreterName pname;
   }) interpreters;
 
-  toMarkdown = data:
+  toMarkdown =
+    data:
     let
       line = package: ''
-        | ${package.pname} | ${join ", " package.aliases or [ ]} | ${package.interpreter} |
+        | ${package.pname} | ${lib.concatStringsSep ", " package.aliases or [ ]} | ${package.interpreter} |
       '';
     in
-    join "" (map line data);
-
-  join = lib.strings.concatStringsSep;
+    lib.concatStringsSep "" (map line data);
 
 in
-''
+writeText "python-interpreter-table.md" ''
   | Package | Aliases | Interpeter |
   |---------|---------|------------|
   ${toMarkdown result}