doc/manual: Render unreleased release notes on master

This renders the rl-next notes when `officialRelease = false`, which
corresponds to the case where we're not on a release branch.

Previously we had disabled this behavior because changelog-d is
somewhat of a heavy dependency, being the only Haskell package.
However, we now have new circumstances that topple the tradeoff.

- We render `master` docs to https://nix.dev/manual/nix/development/release-notes/rl-next.html

- `.#manual` is a separate build now, so `nix build nix/foo` is
  not affected by the increased closure of build input outputs.

Because of these factors, I believe adding this functionality back
is more valuable, as we can use it to

- Previous release notes

- Showcase the upcoming release to the community

doc/manual/package.nix

@@ -11,6 +11,8 @@
   python3,
   rsync,
   nix-cli,
+  changelog-d,
+  officialRelease,
 
   # Configuration Options
 
@@ -44,16 +46,24 @@ mkMesonDerivation (finalAttrs: {
   ];
 
   # Hack for sake of the dev shell
-  passthru.externalNativeBuildInputs = [
-    meson
-    ninja
-    (lib.getBin lowdown-unsandboxed)
-    mdbook
-    mdbook-linkcheck
-    jq
-    python3
-    rsync
-  ];
+  passthru.externalNativeBuildInputs =
+    [
+      meson
+      ninja
+      (lib.getBin lowdown-unsandboxed)
+      mdbook
+      mdbook-linkcheck
+      jq
+      python3
+      rsync
+      changelog-d
+    ]
+    ++ lib.optionals (!officialRelease) [
+      # When not an official release, we likely have changelog entries that have
+      # yet to be rendered.
+      # When released, these are rendered into a committed file to save a dependency.
+      changelog-d
+    ];
 
   nativeBuildInputs = finalAttrs.passthru.externalNativeBuildInputs ++ [
     nix-cli