Models docs URL migration — waves → models, version dropdown flattened

The Models product now lives under /models/ on the docs site, replacing the older /waves/ prefix. The version dropdown is also gone. There is only one active version (v4.0.0), and it is served from a flat URL — /waves/v-4-0-0/documentation/text-to-speech-lightning/streaming becomes /models/documentation/text-to-speech-lightning/streaming.

Every old URL 301-redirects to its new equivalent. Legacy v2.2.0 and v3.0.1 bookmarks (retired earlier) also land on the same flat /models/* paths. Bookmarks, external links, and search-indexed pages keep resolving.

API endpoints and SDK identifiers are unchanged. POST /waves/v1/tts, POST /waves/v1/chat/completions, wss://api.smallest.ai/waves/v1/stt/live, and the waves OpenAPI namespace on the SDK all still work exactly as before. Only docs URLs moved.