feat: init docs

1. Add scripts `scripts/gen-doc.sh` to generate documents automatically
to `docs/api/docs.md`
2. Add `make generate-doc`
3. Add docs in `docs/main.md`

api/v1/doc.go

@@ -0,0 +1,16 @@
+// Copyright 2020 Yunion
+//
+// 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.
+
+// +groupName=onecloud.yunion.io
+package v1