doc: provide more examples for parameter validation

[Haswf]

Is your feature request related to a problem? Please describe.

Hertz integrates with go-tagexpr for paramater binding and validation. However, go-tagexpr itself is poorly documented, therefore it becomes difficult for people who are new to hertz to add their first vd tag.

Describe the solution you'd like

I would recommand add examples for the following tags:

• length validation for string and list len($)>0
• regex pattern match for string regexp('^\\w*$')"
• value validation for numertic field $>0
• validation for pointer field num==nil || num>0
• validation for enum types type=="hello" || type == "world"
• custom error message msg:'C must be false when S.A>0'"

I have used these tags extensively in production and these tags should cover a widely range of use cases.
Also, we could provide a example of defining vd tags in idl, this should also helps.

Describe alternatives you've considered

Alternative, we can add these examples to reference if it doesn't fit into the scope of Binding and validate

Additional context

I have been used vd for almost 2 years. I constantly found myself confused and frustared with go-tagexpr's documentation. Hopefully, with these examples, hertz users can get started with vd with much less pain.

[Haswf]

related issue: #632

[Duslia]

Could you help to add more examples? @Haswf

[FGYFFFF]

@Haswf We have refactored the parameter binding and validation capabilities (#541), and the new validation function will use "go-playground/validator".

The "go-tagexpr" will be moved to "hertz-contrib".

You can write a new example after the new version is released