Add Detailed HashiCAT Documentation

It would be great to have detailed documentation about HashiCAT, what it does and how it works.

This could be somewhat along the lines of the [Raft documentation](https://raft.github.io/) (idea by @findkim). Maybe not as detailed at the start but something that can help others who are new to the project understand it more easily.

Start by reviewing the repository's existing documentation and implementation to understand HashiCAT's purpose and behavior. Document what HashiCAT does and how it works in a beginner-friendly form, using the linked Raft documentation as a reference for structure; done means the project is substantially easier for newcomers to understand.

Written by the indexing model from the issue text.