docs: improve IA for AI agent builders — add developer quick start

[ekline]

Summary

A persona-based information architecture review identified that the AI Agent Builder persona — Fastn's primary target audience — has no code-first entry point in the documentation. The entire Python SDK, CLI, and MCP programmatic setup are undocumented in docs/. This PR adds a Developer Quick Start page and places it as the first item under "Getting started" in the navigation.

Persona context

AI Agent Builder — An intermediate-to-senior developer integrating Fastn's SDK or MCP server into an AI agent to give it access to 250+ enterprise tools. Their goal is to install the SDK, configure auth, and make their first tool call as fast as possible.

Top 3 findings from IA review

Rank	Priority	Persona	Issue	Affected area	Proposed fix
1	High	AI Agent Builder	No SDK/CLI/MCP developer documentation exists — the primary persona has no code-first entry point	Entire docs site — missing section	Create a "Developer quick start" page with SDK install, auth setup, and first tool call; add to top-level nav
2	High	Automation Builder	Quick Start Guide buried below reference content in Flows nav; "Your First Automation" links to UCL tutorials instead of Flows Quick Start	SUMMARY.md, your-first-automation.md	Reorder Flows nav to put Quick Start first; add Flows Quick Start link to "Your First Automation"
3	Medium	All personas	Only 10% of pages have internal cross-links — personas can't navigate between related concepts without sidebar	All 88 pages	Add contextual cross-links to key pages (triggers→scheduling, connectors→auth, UCL→SDK)

What this PR fixes

Finding #1: Creates getting-started/developer-quick-start.md covering:

• pip install fastn-ai and configuration (env vars + CLI login)
• First tool call via dynamic proxy (fastn.slack.send_message)
• Semantic tool discovery with get_tools_for()
• Full agentic workflow examples (OpenAI + Anthropic Claude)
• Multi-tenant support with tenant_id
• Troubleshooting table for common errors
• Cross-links to UCL embedding, MCP server, Flows, and multitenancy docs

Also updates SUMMARY.md to place "Developer Quick Start" as the first item under "Getting started", giving the AI Agent Builder persona immediate visibility.

Remaining items (follow-up work)

• Tutorial: Your first embedded integration widget #2: Reorder Flows nav so Quick Start Guide appears before Flow Setup Essentials; update "Your First Automation" to link to the Flows Quick Start
• Tutorial: Build your first flow #3: Add contextual cross-links across key pages (currently only 9 of 88 pages have internal links)

How to verify

1. From the perspective of an AI agent builder visiting the docs for the first time:
   • Open the sidebar → "Developer Quick Start" should be the first item under "Getting started"
   • The page should take them from pip install to a working LLM tool-calling loop in 6 steps
2. Verify all internal links in the new page resolve (4 cross-references to existing pages)
3. Verify code examples match the SDK source (fastn-sdk/python/fastn/client.py)