Assemble experimental feature docs outside of Nix itself

Instead of constructing a markdown list in C++ (which involved all sorts
of nasty string literals), export some JSON and assemble it with the
manual build system.

Besides following the precedent set with other dumped data, this is a
better separate of content and presentation; if we decide for example we
want to display this information in a different way, or in a different
section of the manual, it will become much easier to do so.

doc/manual/generate-xp-features.nix

@@ -0,0 +1,11 @@
+with builtins;
+with import ./utils.nix;
+
+let
+  showExperimentalFeature = name: doc:
+    squash ''
+      - <span id="xp-feature-${name}">[`${name}`](#xp-feature-${name})</span>
+
+      ${indent "  " doc}
+    '';
+in xps: indent "  " (concatStringsSep "\n" (attrValues (mapAttrs showExperimentalFeature xps)))