DEV: Add site.json to api docs (#14249)

Documenting the site.json api endpoint. This endpoint is often used as a
way to get all of the categories and subcategories in a single api call.

spec/requests/api/site_spec.rb

@@ -0,0 +1,34 @@
+# frozen_string_literal: true
+require 'swagger_helper'
+
+describe 'site' do
+
+  let(:admin) { Fabricate(:admin) }
+
+  before do
+    Jobs.run_immediately!
+    sign_in(admin)
+  end
+
+  path '/site.json' do
+
+    get 'Get site info' do
+      tags 'Site', 'Categories'
+      operationId 'getSite'
+      description 'Can be used to fetch all categories and subcategories'
+      consumes 'application/json'
+      expected_request_schema = nil
+
+      produces 'application/json'
+      response '200', 'success response' do
+        expected_response_schema = load_spec_schema('site_response')
+        schema expected_response_schema
+
+        it_behaves_like "a JSON endpoint", 200 do
+          let(:expected_response_schema) { expected_response_schema }
+          let(:expected_request_schema) { expected_request_schema }
+        end
+      end
+    end
+  end
+end