feat(sandbox): platform hooks for usage limits (runtime metering + concurrency gate) - #210

Changed pages include:

• index.html - view
• 404.html - view
• guide/index.html - view
• guide/ai.html - view
• guide/app-types.html - view
• guide/auth.html - view
• guide/database.html - view
• guide/deployment.html - view
• guide/env-vars.html - view
• guide/jobs.html - view

There are more changes not included in the list