feat(page): add PageType to Page model

this commit add a PageType enum containing supported type for writing
documentations like MD, SWAGGER, etc

Breaking changes:
- type attribute in Page.java is now of type PageType

src/main/java/io/gravitee/repository/management/model/PageType.java

@@ -1,3 +1,18 @@
+/**
+ * Copyright (C) 2015 The Gravitee team (http://gravitee.io)
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *         http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
 package io.gravitee.repository.management.model;
 
 /**