Add Detailed HashiCAT Documentation

[BeenzSyed]

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 (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.

[eikenb]

Thanks for taking the time to file this issue @BeenzSyed!

The current plan is to focus on the generated godocs for documentation, with a particular focus on examples. Thought a separate overview/intro type page might also be good with pointers to the repo and the godocs at some point.

The raft site is more about the raft algorithm than about our raft library, so I'm not sure it makes for a good example to follow in this case.