Suggestion about "fluid's documents for developers".

[gongweibao]

Kubernetes documents and Go blogs are very detailed and helpful for developers.We can learn from their's document structure and layout.
First, add concepts in fluid such as program block variable scope compile time runtime and so on. It can be an overview or index of our's develop documents.
Second, add tasks such as how to add operators(we already have one and may need to organized by index) and so on.
Third, add some blogs show detailed thinks just like Go blogs, some is discussed here.