[Non code Contribution] Auto-generate cloud resource reference documentation

KubeVela is a powerful CI/CD platform that let you import and manage all kinds of cloud resources from almost every cloud vendor. To all these cloud resources, a clear doc of each that guides us to smoothly use it is necessary and very friendly to the community.

Regarding tons of cloud resources adding into the KubeVela platform, maintain their accordingly docs is a major task. It'll be landed even more faster with the help from you. Thanks for contribution!

### Missing cloud resources doc (up to date of 2020/02/25, KEEP UPDATING):
- [x] [Azure MariaDB](https://github.com/oam-dev/catalog/blob/master/addons/terraform-azure/definitions/terraform-azure-database-mariadb.yaml), example, adding up the missing ones at below
- [ ] Azure/GCP/AWS/TencentCloud/AlibabaCloud XXX
- [ ] ....

### How-to in simply using Vela CLI(Make [Azure MariaDB](https://github.com/oam-dev/catalog/blob/master/addons/terraform-azure/definitions/terraform-azure-database-mariadb.yaml) as a guide):

1. run a Vela CLI command `vela addon enable terraform-azure` that installs Azure Terraform provider for you
2. using `vela def list` to check the official name of MariaDB
3. using `vela def doc-gen azure-database-mariadb -n vela-system` to generate the doc
4. you'll find 2 azure-database-mariadb.md files that are in Chinese and English at the current directory
5. go submitting a [PR](https://github.com/oam-dev/kubevela.io/compare) to [Supported Cloud Resource list](https://kubevela.io/docs/next/end-user/components/cloud-services/cloud-resources-list)
6. comment in this issue to let the community know and Hooray!

Start with the Azure MariaDB definition and example, then run `vela addon enable terraform-azure`, `vela def list`, and `vela def doc-gen azure-database-mariadb -n vela-system` to understand the documented workflow. Choose a missing cloud resource, generate its Chinese and English documentation, and update the Supported Cloud Resource list with a PR.

Written by the indexing model from the issue text.