[Doc]: Please add CODE_OF_CONDUCT.md document

[longdafeng]

Description

The code_of_conduct.md is beneficial for developers in the community as it establishes a standard for the community to follow. Similar documents can be found in other open source projects.

Documentation links

Please refer to https://github.com/oceanbase/oceanbase/blob/master/CODE_OF_CONDUCT.md

Additional context

No response