docs: refresh README + point logo at marketing landing

[ozpool]

Summary

Two small things, one PR:

1. Logo link — TopNav's perplex mark now points at `/` (the marketing landing with the "Signed, settled, sovereign" hero) instead of `/markets`. Brand mark should always feel like "go home"; on the trade page, the previous /markets target was already where the user came from, but the root marketing page is what really reads as the home surface.
2. README refresh — replaces the three-terminal manual quickstart with `./scripts/dev-up-all.sh` as the primary path, documents the live Pyth Hermes oracle relayer, adds a troubleshooting block, and lists the new `funding.{marketId}` WS channel.

README diff highlights

• Quickstart uses dev-up-all by default; manual flow kept as "advanced".
• Demo wallet snippet (anvil Phase 0: Cargo workspace + perplex-core crate (types, margin, eip712, api) #5) + 100k USDC dev-seed note.
• Live-oracle verification trick (`curl /v1/markets | jq` twice 2s apart).
• Troubleshooting block: empty Position panel, JWT ExpiredSignature, "static" pill, etc.
• WS channels table gains `funding.{marketId}` + the framed-op subscribe example.
• Capabilities list adds live Pyth marks, cross-margin aggregates, slippage-aware liq, self-trade prevention, 8h funding.

Test plan

• pnpm -C web typecheck clean
• Open /trade, click perplex logo → lands on / with the Hero
• Skim updated Quickstart against a fresh clone (out of scope here)