docs(examples): add webhook example; fix MCP README placeholders; improve example docs

[glennreyes]

Hey team, thanks for this project! While reading the README and trying the
examples, I ran into a few small papercuts:

• The MCP section had placeholder URLs and an example server name that could
  trip up first‑time setup.
• No tiny copy‑paste webhook example, even though webhooks are a common path.
• The examples README referenced a generic example.ts that doesn’t exist.

So I made a small docs/examples pass:

• Added examples/webhookExpress.example.ts: a minimal Express server using
  @polar-sh/sdk/webhooks with express.raw for signature verification.
• Updated examples/README.md with accurate run commands and a short list of
  available examples.
• Added convenience scripts in examples/package.json (start:orgs,
  start:webhooks) and dev deps used by the example.
• Pointed README’s MCP binary download to polarsource/polar-js, clarified
  {tag}/releases, and renamed the sample server id to Polar.

All changes are in docs/examples (non‑generated) to align with CONTRIBUTING. If
you prefer different file names or placement, happy to adjust. Thanks again!