docs: add WebSocket usage + reorganize README/docs (#183)

README: overview at top, extras table, Configuration->Authentication,
3-tab Quickstart (Sync/Async/WebSocket), Clients table, REST reference
regrouped by product (CEX / Derivatives / Pricing / Alt-data) with the
5 previously-undocumented endpoints (liquidation, open_interest,
margin_borrow, index_price, cex.symbol) folded in, full WebSockets
section (channels, multiplexing, feeds, reconnect/keepalive, lifecycle).

docs: new websocket.md (+ AsyncDatamaxiWS reference) in nav after Async
Client. docs/index.md is a symlink to README so the WS content flows to
the docs home automatically.

Verified: mkdocs build --strict passes.

Author: martinkersner