doc: add layer 2 stuff and dark forest circuit walkthrough

[Yijia-Chen]

[Please describe your pull request here]

Checklist

• [ ] The URL is not already present in the list (check with CTRL/CMD+F in the raw markdown file).
• [ ] Each description starts with an uppercase character and ends with a period.
  Example: solc-js - JavaScript bindings for the compiler.
• [ ] Drop all A / An prefixes at the start of the description.
• [ ] Avoid using the word Solidity in the description.