docs: add Kalshi demo to streaming venue lists

[happykawayigt]

What & why

Add kalshi-demo to the supported venue lists for watchOrderBook, watchOrderBooks, and watchTrades. The demo exchange inherits these streaming methods from Kalshi, so the API reference should advertise the same capability.

Closes #1928.

Test plan

• npm run docs:llms
• verified the three streaming pages each contain exactly one supported-venues line including kalshi-demo
• git diff --check

AI assistance

AI assistance was used to inspect the documented capability path, make this scoped documentation update, and run validation. I reviewed the final diff and results before submission.