feat: Update configuration for Rails v8.1 doc

Author: toshimaru

_config.yml

@@ -2,11 +2,13 @@ title: RailsDoc(β)
 description: Ruby on Rails API Documentation.
 url: https://railsdoc.github.io
 source: src
-default_rails_version: "8.0.3"
+default_rails_version: "8.1.0.beta1"
 rails_versions:
+  "8.1":
+    specific_version8.1.0.beta1: "8.1.0.beta1"
+    latest: true
   "8.0":
     specific_version: "8.0.3"
-    latest: true
   "7.2":
     specific_version: "7.2.2"
   "7.1":
@@ -33,10 +35,15 @@ defaults:
   - scope:
       path: ""
     values:
-      version: 8.0
+      version: 8.1
       image: https://avatars.githubusercontent.com/u/4223
       toc: true
       root_path: "/"
+  - scope:
+      path: "8.0"
+    values:
+      version: 8.0
+      root_path: "/8.0/"
   - scope:
       path: "7.2"
     values: