Automatically document builtin constants

This is done in roughly the same way builtin functions are documented.

Also auto-link experimental features for primops, subsuming PR #8371.

Co-authored-by: Eelco Dolstra <edolstra@gmail.com>
Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>
Co-authored-by: Valentin Gagarin <valentin.gagarin@tweag.io>

doc/manual/generate-builtins.nix

@@ -1,24 +1,28 @@
 let
-  inherit (builtins) concatStringsSep attrNames;
+  inherit (builtins) concatStringsSep attrValues mapAttrs;
+  inherit (import ./utils.nix) optionalString squash;
 in
 
 builtinsInfo:
 let
-  showBuiltin = name:
+  showBuiltin = name: { doc, args, arity, experimental-feature }:
     let
-      inherit (builtinsInfo.${name}) doc args;
+      experimentalNotice = optionalString (experimental-feature != null) ''
+        This function is only available if the [${experimental-feature}](@docroot@/contributing/experimental-features.md#xp-feature-${experimental-feature}) experimental feature is enabled.
+      '';
     in
-    ''
+    squash ''
       <dt id="builtins-${name}">
         <a href="#builtins-${name}"><code>${name} ${listArgs args}</code></a>
       </dt>
       <dd>
 
-        ${doc}
+      ${doc}
+
+      ${experimentalNotice}
 
       </dd>
     '';
   listArgs = args: concatStringsSep " " (map (s: "<var>${s}</var>") args);
 in
-concatStringsSep "\n" (map showBuiltin (attrNames builtinsInfo))
-
+concatStringsSep "\n" (attrValues (mapAttrs showBuiltin builtinsInfo))