"Going into production" docs update

[infomiho]

This is the first PR in the deployment docs series, it covers:

• general structure
• self-hosting Wasp apps

The idea is to have general concepts of deployment explained first: Wasp app structure (it's React + Node.js + database), env variables, database (migrations, connecting to the DB) and then describe deployment methods. There are some CI/CD things I'd like to mention and then some extras like custom domains, CDNs etc.

I'll be branching from this branch when I'll write the other parts of the deployment docs:

• Wasp app structure overview Deployment introduction #2380
• Env variables Update env variables docs #2375
• Database Production database docs #2376
• Client deployment
• CI/CD
• Extras