[Reduce Manual Work] Modify readme.md for extension introduction

[fengzhou-msft]

Issue:

1. Add a link to service documentation so users can get to know more about the service managed by the extension. The link can be the same as the one in https://github.com/Azure/azure-cli-extensions/blob/master/src/service_name.json. (service_name.json is based on command group, for some extension it may be hard to map).

2. When the command interface is changed through readme.az.md configuration in regeneration, it looks like README.md of the extension does not change.

Impact:
Add a link will save little time but useful for users. Automatically updating the readme in regeneration should save developers much work.