docs: add Python SDK quickstart tutorial

[ekline]

Context

This tutorial addresses the #1 gap identified through a tutorial coverage audit that included competitor analysis, persona needs assessment, and feature coverage review.

The problem: Fastn has a full-featured Python SDK (fastn-ai on PyPI) with 250+ connectors, semantic tool discovery, and LLM agent integration — but zero tutorial coverage for developers who want to use it programmatically. All existing tutorials are UI/flow-builder focused. Every major competitor (Composio, Nango, Paragon, Merge) has an "install → authenticate → first API call in 5 minutes" tutorial as a core onboarding path.

Top 3 gaps identified

Rank	Priority	Gap	Source	Personas served
1	High	Python SDK quickstart — no tutorial for install, auth, first tool call	All 4 competitors have this; developer persona blocked	AI engineers, backend developers
2	High	MCP server setup with Claude Desktop/Cursor — no developer-focused tutorial	3/4 competitors cover this; AI engineer persona blocked	AI engineers
3	Medium	CLI getting started — no tutorial for fastn login, fastn connector, fastn flow	Developer persona needs; new feature without tutorial	Backend developers

What this PR adds

• New file: getting-started/sdk-quickstart.md — A 5-minute tutorial that walks developers through:
  1. Installing the fastn-ai package
  2. Configuring credentials (env vars, constructor params, config file)
  3. Sending a Slack message via fastn.slack.send_message()
  4. Error handling with SDK exceptions
  5. Semantic tool discovery with get_tools_for()
  6. Building an LLM agent with OpenAI integration
  • Includes troubleshooting table for common issues
• Updated: SUMMARY.md — Added navigation entry under "Getting started"

Research sources

• Competitors audited: Composio (docs.composio.dev), Nango (docs.nango.dev), Paragon (docs.useparagon.com), Merge (docs.merge.dev)
• Personas served: AI engineers integrating Fastn with LLM agents, backend developers building programmatic integrations
• Code verified against: fastn-sdk/python/fastn/client.py, config.py, connector.py, exceptions.py, _constants.py, pyproject.toml, and example files

Remaining gaps (follow-up work)

• Tutorial: Your first embedded integration widget #2: MCP server setup tutorial for Claude Desktop / Cursor (how-to guide)
• Tutorial: Build your first flow #3: CLI getting started tutorial for fastn login, fastn connector, fastn flow

Quality review

• Completeness: All required tutorial sections present (intro, prerequisites, 6 steps, summary, next steps, troubleshooting)
• Technical accuracy: All code examples verified against SDK source — method signatures, env var names, package name, exception classes all confirmed
• Style compliance: Active voice, second person, present tense, verb-first procedures, no banned phrases, sentence case headings
• Links: All 3 internal links verified to resolve to existing files
• Grade: A- (only note: H3 headings under H1 match existing repo convention)

How to verify

1. Check out this branch
2. Run the GitBook dev server or preview
3. Navigate to Getting started → Python SDK Quickstart
4. Walk through the tutorial steps with a Fastn account and verify the code examples work