ヘルプを Wiki から分離 (#424) (#430)

画面上でのテストは完了．
移行にす早さが求められるので，準備ができたらマージする．

Reviewed-on: #430
Co-authored-by: miteruzo <miteruzo@naver.com>

backend/spec/db/tag_description_migration_spec.rb

@@ -5,11 +5,11 @@ RSpec.describe 'Tag description migration identity' do
   it 'preserves every revision ID and repeated body in an independent sequence' do
     user = create_member_user!
     tag_name = create(:tag_name, name: 'description_migration')
-    page = Wiki::Commit.create_content!(
+    page = Wiki::Commit.create_legacy_content!(
         tag_name:, body: 'A', created_by_user: user)
     revisions = [page.current_revision]
     ['A', 'B', 'B', 'A'].each do |body|
-      revisions << Wiki::Commit.content!(
+      revisions << Wiki::Commit.legacy_content!(
           page:, body:, created_user: user, base_revision_id: revisions.last.id)
     end
     tag = create(:tag, primary_tag_name: tag_name)