Write guide about running on Serverless platforms

[duncanmcclean]

This has come up a couple of times before, but it'd be nice to have a generalised guide to point folks to if they want to use a serverless platform, like Laravel Vapor or DigitalOcean App Platform.

There quite a lot of gotchas you need to be aware of when considering going "serverless" with a Statamic site:

• Serverless platforms often have read-only filesystem, so you can't write back to flat-files, so content needs to be stored in a database.
• By default, Statamic uses the file drivers for caching and sessions. You'll need to use database (or redis) drivers for this instead.
  • Laravel Vapor does this for you out of the box.
• When using a platform like DigitalOcean's app platform, you will need to configure trusted proxies.
  • Laravel Vapor does this for you out of the box.