docs: rewrite the documentation for v6

Rewrite every guide for the redesigned app, add guides for new features (documents, editor modes, check, cover letters, assistant, applications, self-hosting upgrade and environment reference), remove v5-only pages with redirects, and replace every screenshot.

docs/spec.json

@@ -1,7 +1,7 @@
 {
 	"info": {
 		"title": "Reactive Resume",
-		"version": "5.3.2",
+		"version": "6.0.0",
 		"description": "Reactive Resume API",
 		"license": {
 			"name": "MIT",