DOC: Add error message + note/tip about re-running vak prep if splits fail

[NickleDave]

Is your feature request related to a problem? Please describe.
Sometimes vak prep can fail to find dataset splits as described here
https://forum.vocalpy.org/t/solving-valueerror-could-not-find-subsets-of-sufficient-duration-in-less-than-5000-iterations/52/9

Because of the current algorithm this can just be luck of the draw, and simply re-running can solve the problem. But this is not obvious.

Describe the solution you'd like

• add note to error message saying "try re-running"
• add tip/hint to docs that better explains this

We should for now add a tip/hint to try re-running vak prep when this happens. Should be in

• edit: in the howto/howto_prep_annot page!
  • (formerly: a detailed vak prep vignette)
• troubleshooting
• linked to from FAQs
• elsewhere?

Describe alternatives you've considered
better split algorithm! as in #299

Additional context
Might be worth providing concrete examples like similar to those Mizuki provided