Add use-case/cookbook style documentation

[t3hmrman]

While the sheesy documentation is pretty fantastic already, I felt there was a little to be desired as far as the various vault pages, in particular vault partitions.

I was looking for basically group-like support -- being able to share a secret with a group of users but not necessarily a different group, more concretely to be able to differentiate secrets for staging/production. When looking at the vault partitions add documentation it was unfortunately a little unclear

Since what I was looking for is likely a very common usecase, it might be nice to explain it directly, either on the add page itself or in a separate "recipes" or "use case examples" or something section.

[Byron]

Thanks a lot for letting me know! I think you are absolutely right, documentation about this feature in particular is lacking, even though your initial assessment is right: It's exactly what you want to model groups. These days I am rather unmotivated to work on any of my open-source projects, but my mere reply here shows it's slowly returning :). With every issue raised here and increased demand, I will grow happier to finish the documentation, which to this date is on my todo-list.

[t3hmrman]

No problem, thank you for the work! I was actually planning on writing something like this in rust myself after I'd become an avid user of git-crypt but thanks to your project i don't have to (I had just started thinking of how to implement groups and do all the rehashing that would likely be necessary, etc)!

Also don't worry, I plan to contribute some code -- take care of yourself and the motivation will return eventually! Hopefully soon I can send something and you can sit back and review! :)