docs: restructure README around getting started

[devin-ai-integration]

Summary

Rewrites the top of the README so the npm page for @linqapp/sdk reads like a product page instead of a Stainless boilerplate dump.

What's new above the fold: centered header with tagline + npm/downloads/license/TS badges, an About Linq paragraph, a numbered Getting Started (get a token at dashboard.linqapp.com/api-tooling → install → send a message), Documentation, a Resources table mapping each client.* namespace to what it does, and a Webhooks section (previously the README never mentioned webhooks despite client.webhooks.unwrap shipping in the SDK):

const event = client.webhooks.unwrap(rawRequestBody, { headers: requestHeaders });
if (event.event_type === 'message.received') { ... }

Also: examples now use client.messages.create with real text parts rather than client.chats.create({ message: {} }) (the generated placeholder that sends nothing meaningful), MCP Server and Advanced Usage moved below the useful content, empty ## Frequently Asked Questions heading dropped, and Issues/License sections added. Everything below "Handling errors" is unchanged.

Verification: all README snippets were extracted into a scratch example and typechecked (tsc --noEmit), then deleted; ./scripts/lint passes (prettier, eslint, build, attw, publint).

Caveat: README.md is generator-managed. Per CONTRIBUTING, manual edits persist across generations but can conflict — worth mirroring the intro into the Stainless config if it starts fighting us.

[devin-ai-integration]

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