add more documentation about what gather does, and what the fields are.

[ctb]

as we explore the min-set-cov analysis, we're coming up with some interesting viz, e.g. #1225, and adding new output fields #1219. I think Appendix A, How sourmash gather works, could use some sprucing up as our new understanding evolves.

Specific ideas:

• add some of our new figures! gathergram, in particular.
• add some real examples, maybe
• document output fields in a bit more detail

[ctb]

ref #1789 and #1799

[ctb]

there's a lot of updates here -

The new "lemonade" tutorial added in #2158 provides some discussion, as does the STAMPS 2022 lecture on assembly-free data analysis.

ref #2170 - use detection terminology.

still lots of room for improving the actual documentation 😆