Enhance/add description

[freedeaths]

Why are these changes needed?

Enhance backend to return the description and examples.

Related issue number

Backend part of #75

Checks

• [x] I've included any doc changes needed for https://github.com/pingcap/LinguFlow/.
• [ ] I've added tests (if relevant) corresponding to the changes introduced in this PR.
• [ ] I've made sure all auto checks have passed.

[freedeaths]

@lucklove @shhdgit PTAL

@lilyjazz You can add the fields like here.