Write a lot, like an actual lot. of documentation

Co-authored-by: William Oldham <github@binaryoverload.co.uk>

content/4.backend/0.introduction.md

@@ -3,4 +3,20 @@ title: 'Introduction'
 ---
 # Introduction to the backend
 
-// EXPLAIN WHAT THE BACKEND DOES
+The backend is essentially just an account server. There is not much more to it.
+
+## Metrics
+
+The backend exposes prometheus metrics, it can be accessed on `/metrics`.
+
+## Security
+
+Optionally, there are a few security settings:
+- Recaptcha support, the server can verify Recaptcha v3 tokens on register and login.
+- Ratelimits, Some expensive endpoints have ratelimits, but only when enabled. This requires an additional redis connection.
+
+## Migrations
+
+To run migrations, You can use the command `pnpm migration:up` inside the docker container.
+
+Alternatively, you can enabled `postgres.migrateOnBoot` and it will be automatically migrated on boot.