doc: add document for various admission pickers

There are various admission pickers for different cases, this chapter will introduce them.

• What is the admission picker?
• What does the admission picker do?
• How many admission pickers are provided in foyer default?
  • How can users use them in different cases?
• How can users create a new admission pickers?

extra work

• Add version 0.13
• Enable version next

The chapter will be put in design section.