Add "Apply Your License Key" self-hosting guide (#3077)

Adds a dedicated self-hosting page documenting how to apply an
Enterprise license key, and cross-links it from the related docs.

New page: self-hosting/configuration/license.mdx — "Apply Your License
Key"
Set NEXT_PRIVATE_DOCUMENSO_LICENSE_KEY (Docker Compose / docker run /
.env tabs)

apps/docs/content/docs/self-hosting/configuration/meta.json

@@ -2,6 +2,7 @@
   "title": "Configuration",
   "pages": [
     "environment",
+    "license",
     "database",
     "email",
     "storage",