Add document for API usage and examples

[yunfengzhou-hub]

Clink needs more documentation and examples about the usage of Clink APIs so as to improve Clink's readability and usability. It is equally important to setup conventions for the whole documentation system for Clink's infrastructure and operators.

Possible implementation details of this issue could be to

• Add a docs/ folder containing markdown files of user guides
• Add various Clink programs as examples
• Add more detailed description directly to APIs, like Flink has done to org.apache.flink.table.api.Table.