docs: add WebSocket usage + reorganize README/docs

[martinkersner]

Summary

Documents the WebSocket client (previously undocumented in both the README and the docs site) and reorganizes the README/docs so it reads like a modern SDK guide. Moderate reorg — no code changes.

README

• Overview up top (REST / WebSocket / Sync-or-async at a glance) + extras table ([async], [ws]).
• Configuration → Authentication; env-var note kept.
• 3-tab Quickstart: Sync | Async | WebSocket.
• New Clients table.
• REST API Reference regrouped by product: CEX / Derivatives and Leverage / Pricing and Cross-Exchange / Alternative Data — and folds in 5 endpoints that were undocumented in the README (liquidation, open interest, margin borrow, index price, cex.symbol), with signatures taken from the existing per-endpoint docs pages.
• New full WebSockets section: install, quickstart, channel table (9 accessors + param formats + plan tier), multiplexing/msg["s"] filtering, firehose feed, reconnect/keepalive, lifecycle, message shapes.

docs

• New docs/websocket.md (prose + auto-generated AsyncDatamaxiWS reference), added to the nav after Async Client.
• docs/index.md is a symlink to ../README.md, so the reorganized content (incl. WebSockets) flows to the docs home automatically — left intact.

Tests

• mkdocs build --strict passes clean (only a pre-existing INFO about the LICENSE relative link).
• Confirmed the WebSocket page, the AsyncDatamaxiWS reference block, and the nav entry all render.
• Sanity-checked the WS client at import: all 9 accessors + param_format resolve.
• No source changes, so the pytest suite is unaffected.

Known failures

None.