[M5+] add documentary Cargoxx.toml + fix LinkdbUnknownPackage hint formatting

src/linkdb/database.cpp

@@ -69,8 +69,9 @@ auto Database::resolve(const std::string& package, const std::string& version,
     return std::unexpected(util::Error{
         util::ErrorCode::LinkdbUnknownPackage,
         std::format("package '{}' has no known CMake link recipe", package),
-        "run `cargoxx add {0}` to discover and verify a recipe via the resolver, "
-        "or add a manual recipe via `cargoxx linkdb add`",
+        std::format("run `cargoxx add {}` to discover and verify a recipe via "
+                    "the resolver, or add a manual recipe via `cargoxx linkdb add`",
+                    package),
         std::nullopt, std::nullopt,
     });
 }