Write a lot, like an actual lot. of documentation

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

content/4.backend/3.changelog.md

@@ -1,6 +1,13 @@
 ---
 title: 'Changelog'
 ---
-# Version 1.0.0
+# Version 1.1.5
 
-Changelog goes here
+Initial version of the backend.
+
+- Prometheus metrics endpoint
+- Account creation/deletion endpoints
+- Endpoints for importing old account data
+- Endpoints for syncing data
+- Ratelimit system
+- Captcha system