[Feature]: Revamp docs to be more comprehensive

[forsyth2]

How will this affect the next version number?

Small improvement (increment PATCH version)

Is your feature request related to a problem?

zstash documentation could be made more up-to-date and comprehensive -- especially documentation for developers on how the code works.

Describe the solution you'd like

• Revamp zstash documentation to be more organized and comprehensive.
• Include a user guide and dev guide.

Describe alternatives you've considered

No response

Additional context

• This is the equivalent task of Revamp docs to be more comprehensive zppy#839
• Ideally, this should be done after some key code quality tasks are resolved, so they can be included in the developer guide documentation:
  • Add performance profiling infrastructure #427
  • Refactor zstash for improved maintainability #435

[forsyth2]

Also see #262, #301, #328, #375 for more documentation ideas.