docs(readme): product-focused funnel rewrite

Restructures the README to onboard a broad Ponder audience: a proof strip up top, a 2-minute quickstart
against the free public Portal (framed as capacity-limited), the one-line migration, then the depth.

- Rewrites "Why it's faster" to sell why-fast + why-a-fork, dropping the internal RPC-shim post-mortem.
- Corrects the Portal framing to match sqd.dev / docs.sqd.dev (an HTTP access layer to onchain data, not
  a "columnar data lake") and the EVM network count (130+, not 300+).
- Adds "Going to production" (dedicated Portal — talk to us; self-served tiers coming soon) and
  "Contributing, feedback & bug reports" (GitHub issues + SquidDevs on Telegram).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: dzhelezov