docs: add Scalekit integration page

[saif-at-scalekit]

Summary

Adds a Scalekit integration page to the platform documentation. Scalekit is auth infrastructure for AI agents that provides a token vault and connector layer for OAuth 2.0 flows, token storage, automatic refresh, and API proxying for 100+ third-party services.

New page contents

• What is Scalekit — overview of the token vault and connector layer
• Prerequisites — Scalekit account, Apify account, Node SDK install, env vars
• Step-by-step guide:
  1. Initialize the Scalekit client
  2. Connect a user's account via getOrCreateConnectedAccount
  3. Authorize via OAuth link with polling loop
  4. Call APIs via executeTool (pre-built tools) or actions.request (direct)
• Complete working example — full Actor code (Notion search)
• LLM agent Actor section — using executeTool in an LLM tool-calling loop
• Available connectors table — 14 common services with connectionName values
• Resources — links to AgentKit docs, quickstart, connectors, example repo

Style guide alignment

• Front matter description uses the "Learn how to..." pattern (140-160 chars)
• Prerequisites formatted with italic item names per existing integration pages
• Third-party disclaimer partial imported and placed after intro section
• Follows Conventional Commits (docs: prefix)

Related

• Notion + YouTube Agent example repo
• Apify Actor per-user OAuth cookbook