chore: add docs subpath (#160)

Co-authored-by: Stanley Jones <stanley@alchemy.com>

Author: fern-support

fern/docs.yml

@@ -1,7 +1,8 @@
 # yaml-language-server: $schema=https://schema.buildwithfern.dev/docs-yml.json
 
 instances:
-  - url: https://alchemy.docs.buildwithfern.com
+  - url: https://alchemy.docs.buildwithfern.com/docs
+    custom-domain: https://alchemy.com/docs
 
 layout:
   searchbar-placement: header