doc: Add FastAPI Quickstart for Python Web Apps

[kishore7snehil]

Description

Add a comprehensive FastAPI quickstart guide for Python web applications to the Auth0 documentation.

This quickstart demonstrates how to integrate Auth0 authentication into FastAPI applications using the Auth0 FastAPI SDK. It includes:

• Step-by-step setup instructions with CLI and Dashboard options
• Complete code examples with modern UI
• Advanced usage patterns (role-based access control, API calls, Redis sessions)
• Comprehensive troubleshooting section
• AI-powered integration prompt for IDE assistants

The guide is added to the web app quickstart navigation alongside Django, Express, and other frameworks.

Changes

• Added /main/docs/quickstart/webapp/fastapi/index.mdx - Complete FastAPI quickstart guide
• Updated /main/docs.json - Added FastAPI to web app quickstart navigation

Checklist

• I have added documentation for new/changed functionality in this PR or in auth0.com/docs
• All active GitHub checks for tests, formatting, and security are passing
• The correct base branch is being used, if not the default branch

[github-actions]

Summary

Status	Count
🔍 Total	8
✅ Successful	5
⏳ Timeouts	0
🔀 Redirected	1
👻 Excluded	2
❓ Unknown	0
🚫 Errors	0
⛔ Unsupported	0

Redirects per input

Redirects in main/docs/quickstart/webapp/fastapi/index.mdx

• [200] https://pip.pypa.io/ | Redirect: Followed 1 redirect resolving to the final status of: OK. Redirects: https://pip.pypa.io/ --> https://pip.pypa.io/en/stable/
  Full Github Actions output