Fix variant URL routing for SDK Reference product

Per the Fern variant model (see server-sdks.yml), page slugs must be
relative to the product slug and must include the tab segment so the
URL resolves as /docs/<product>/<tab>/<variant>/<page>. Our slugs were
including the product prefix (/sdk-reference/...), which Fern was
prepending /sdk-reference/ to anyway, producing the doubled path
/sdk-reference/sdk-reference/typescript/... and leaving the intended
variant URLs (/sdk-reference/reference/<lang>) as 404s.

Regenerated with base_slug: /reference/<lang> so each page now emits
slug: /reference/<lang>/<page> and resolves at the right Fern URL.

Also adds explicit slug: dotnet and slug: cpp to the .NET and C++
variants so Fern's title-derived auto-slugs don't produce paths with
problematic characters (`.net`, `c++`).

Author: Devon-White

fern/products/sdk-reference/cpp/_meta.json

@@ -5,6 +5,6 @@
   "source_repo": "https://github.com/signalwire/signalwire-cpp",
   "format": "mdx",
   "platform": "fern",
-  "base_slug": "/sdk-reference/cpp",
+  "base_slug": "/reference/cpp",
   "item_count": 1678
 }