docs: specify site-url

Fixes 404 page's relative links and styling being relative to `/`.

Cherry-picked:
- a92339d83b
- cdbda982f0

docs/mdbook/default.nix

@@ -6,6 +6,8 @@
   nixosOptionsDoc,
   transformOptions,
   hmOptions,
+  # The root directory of the site
+  baseHref ? "/",
 }:
 with lib;
 let
@@ -238,6 +240,10 @@ let
     # Using pkgs.writeShellScript helps to avoid the "bash: argument list too long" error
     bash -e ${pkgs.writeShellScript "copy_docs" docs.commands}
 
+    # Patch book.toml
+    substituteInPlace ./book.toml \
+      --replace-fail "@SITE_URL@" "${baseHref}"
+
     # Prepare SUMMARY.md for mdBook
     # Using pkgs.writeText helps to avoid the same error as above
     substituteInPlace ./SUMMARY.md \