docs: fix Node version requirement and misleading RPC URL placeholder in Getting Started

[pplmaverick]

Summary

Discovered while following the Getting Started guide to deploy my own prediction market on Arc.

Two inaccuracies in the Getting Started flow that will break onboarding for anyone
following the README literally:

1. Node.js version requirement is wrong

README Prerequisites says "Node.js v18+", but package.json pins "next": "16.2.0".
Next.js 16.2.0's own published package.json declares "engines": {"node": ">=20.9.0"}
(verified against https://registry.npmjs.org/next/16.2.0). A developer on Node 18/19
following the README will hit an unsupported-engine error or undefined behavior at
npm install / npm run dev.

2. RPC URL is not actually "pre-filled" — it's a broken placeholder

README step 2 says: "The Arc Testnet RPC URL is pre-filled." In reality, .env.example
sets NEXT_PUBLIC_ALCHEMY_RPC_URL=https://your-alchemy-rpc-url-here as an active,
uncommented value. Both hardhat.config.ts:45 and lib/chain.ts:21-22 use:

process.env.NEXT_PUBLIC_ALCHEMY_RPC_URL || "https://rpc.testnet.arc.network"

A non-empty placeholder string defeats that fallback. A developer who copies
.env.example → .env.local and only fills in PRIVATE_KEY (as the README instructs)
ends up with npm run deploy and the frontend's RPC calls pointed at the literal bogus
host your-alchemy-rpc-url-here, breaking the guided deploy flow on first run.

Changes

- README.md: correct the Node version requirement to v20.9+ (with a note that it's
required by Next.js 16), and reword the RPC URL description to explain it's optional
and what happens if left unset.
- .env.example: comment out NEXT_PUBLIC_ALCHEMY_RPC_URL's placeholder value (same
pattern already used for the auto-written contract address vars further down in the
same file), so the code's existing fallback to the public Arc Testnet RPC actually
takes effect when a developer doesn't have an Alchemy key yet.

No application code changed — only documentation and the environment template.

Test plan

- [ ] cp .env.example .env.local, fill in only PRIVATE_KEY, run npm run deploy —
confirm it connects to https://rpc.testnet.arc.network instead of failing on
the placeholder host
- [ ] Confirm npm install / npm run dev behave as expected on Node 20.9+