document new SBT zip file output.

[ctb]

per #648, if you ask sourmash index to produce a file with suffix .sbt.zip, it will put the SBT in a zip file. until 4.0, this won't be the default output, but it is extremely useful :)

A few details that might be worth mentioning -

• you can speed up access by unzipping the SBT
• it's not clear if the unzipped version takes up a lot more space, b/c the nodes (internal and signatures both) should all be compressed too, I think?
• we should try to summarize the performance characteristics (detailed in [MRG] add ZipStorage, support loading tree from storage #648) in terms of space, time, and memory tradeoffs, vs the old SBT.

Anything else?

[luizirber]

you can speed up access by unzipping the SBT

Need to measure it better, but doesn't seem to make much difference for now.

it's not clear if the unzipped version takes up a lot more space, b/c the nodes (internal and signatures both) should all be compressed too, I think?

Yup, should take about the same space.

we should try to summarize the performance characteristics (detailed in #648) in terms of space, time, and memory tradeoffs, vs the old SBT.

from #648 (comment)

Level	Size	Time
0	407 MB	16s
1	252 MB	21s
5	250 MB	39s
9	246 MB	1m48s

Anything else?

• Related to upgrade sourmash_databases #970 , update docs and tutorials to use the new zipped prepared SBTs (when they are published)

[ctb]

see #975.

turns out our index documentation in the docs is ...terrible. We should have a whole section on database types, I guess.

[ctb]

(by terrible I mean nonexistent :)