Update grate and 3i docs

[Yaxuan-w]

Thanks to @marcelamelara for the comments about grate and 3i on Issue #183. I'm opening this issue as a TODO to track that. I'll update the documentation for grate and 3i after the implementation is complete.

[Yaxuan-w]

As discussed in PR, my current thought is to add documentations of following aspects,

• high-level implementation
• exit mechanism

would appreciate any suggestions on this