feat(docs): add docs for finite state machine

[Narayanbhat166]

This PR adds the intent state finite state machine docs in payments

Type of Change

• [x] Documentation

Description

Add documentation for intent state finite machine.

Motivation and Context

In order to have clear understanding of the states a payment can be in.

How did you test it?

No testing required. This is a documentation addition.

Checklist

• [x] I formatted the code cargo +nightly fmt --all
• [x] I addressed lints thrown by cargo clippy
• [x] I reviewed the submitted code