Add guide on auto scale with Flame

[josevalim]

It should probably link to world page speed source and explain the difference between async, durable and auto scalable.

[eagle-head]

Hi @josevalim, I'd like to work on this guide. Before starting, I have a few questions to make sure I'm heading in the right direction:

About the reference:

1. The issue mentions linking to the "world page speed source" — I couldn't find this repository (got a 404 on chrismccord/world_page_speed). Is there a replacement repo or example app I should reference instead?

About the scope:
2. Should the guide cover only the conceptual differences between async, durable, and auto-scalable — or also include a hands-on walkthrough (adding FLAME to a Phoenix app, configuring pools, deploying)?
3. Should I mention specific libraries by name (e.g., Oban for durable jobs) or keep it generic ("a job queue library")?
4. Should the guide cover FLAME backends beyond Fly.io (e.g., FLAME.LocalBackend for dev), or focus on the general concept?

About the placement:
5. Where should this guide live? I'm thinking either:

• guides/deployment/auto_scaling_with_flame.md (since it's deployment-related), or
• guides/howto/auto_scaling_with_flame.md (since it's a how-to pattern)

About the audience:
6. Should the guide assume the reader already knows about Task.async and GenServer, or should it briefly introduce those before contrasting with FLAME?

Happy to submit a draft PR early if you'd prefer to review the direction before I flesh it out.

[josevalim]

I am assigning it to Chris, this was mostly written as a reminder for him. He can either tackle it or answer your questions.