Add high-level overview to docs

[forsyth2]

Create a schematic diagram (similar to https://e3sm-project.github.io/zppy/_build/html/main/schematics.html) on how zstash operates, in the HPSS, Globus-HPSS, no-HPSS cases.

[forsyth2]

From @TonyB9000's suggestion on #261: Graphical/pictorial representations of steps depicting tape-archive, local caches, final destinations, etc with create and extract would probably be useful someday, for HPSS and non-HPSS usages

[forsyth2]

Incorporating this into the broader-scope #451.