Remove all Defang Playground references from documentation

[simple-agent-manager]

Context

We are removing the Defang Playground. All references to it in the documentation need to be updated or removed.

Tracked from: https://github.com/DefangLabs/defang-global/pull/23

Checklist

Blog posts

• blog/2025-07-10-june-product-updates.md:48
• blog/2025-04-23-vibe-deploy.md:51
• blog/2024-02-12-announcing-defang-public-beta.md:12
• blog/2024-09-30-september-product-updates.md:26
• blog/2025-06-06-may-product-updates.md:28
• blog/2024-07-01-july-product-updates.md:23
• blog/2025-08-07-defang-ai-devops-agent.md:43
• blog/2025-06-16-crew-ai-sample.md:66
• blog/2025-04-10-easiest-way-to-deploy-django.md:68
• blog/2025-04-10-easiest-way-to-deploy-django.md:70

Concepts

• docs/concepts/pulumi.md:12
• docs/concepts/networking.mdx:109,113
• docs/concepts/accounts.md:10
• docs/concepts/domains.mdx:22,75,77,112
• docs/concepts/managed-llms/managed-language-models.md:49
• docs/concepts/defang-playground.md — REMOVE ENTIRE FILE
• docs/concepts/defang-byoc.md:8,23
• docs/concepts/portal.md:8,13
• docs/concepts/observability.md:17
• docs/concepts/deployments.md:8,21

Tutorials

• docs/tutorials/adding-custom-one-click-deploy.md:12
• docs/tutorials/using-one-click-deploy.md:3,10,46,56
• docs/tutorials/deploy-to-playground.mdx:8 — REMOVE ENTIRE FILE if dedicated
• docs/tutorials/deploy-with-gpu.mdx:12
• docs/tutorials/migrating-from-heroku.md:180
• docs/tutorials/deploy-using-pulumi.mdx:8,127
• docs/tutorials/monitoring-your-services.md:50
• docs/tutorials/deploy-to-your-cloud.mdx:12

Providers

• docs/providers/playground.md — REMOVE ENTIRE FILE

Intro

• docs/intro/what-is-defang.mdx:25
• docs/intro/faq/questions.md:14,69,72

cc @raphaeltm