update docs with pretty pictures from betterplot plugin

[ctb]

the betterplot plugin (#3178 and https://github.com/sourmash-bio/sourmash_plugin_betterplot/) provides some nice additional visualization capabilities to sourmash. We should advertise those on the main sourmash docs, I think.

[ctb]

a start here: https://github.com/ctb/2024-sourmash-example-plots

used for talk: https://osf.io/mkgx4/

[ctb]

also wondering if maybe we want to split the sourmash docs from the sourmash code repository (I thought we had an issue about this somewhere, but can't find it).

disadvantages:

• more complexity
• docs are decoupled from sourmash code releases (but they already kind of are)
  advantages:
• can update them more quickly if we decouple from tests, reviews necessary on main repo;
• makes better sense given the plugin situation
• might allow for more contributors
• enables "the pull request hack" link for docs
• ???