Start documentation of sum_to_zero_vector

[WardBrian]

Submission Checklist

• Builds locally
• New functions marked with <<{ since VERSION }>>
• Declare copyright holder and open-source license: see below

Summary

Closes #804.

I think I need some help from @spinkney @bob-carpenter filling out the documentation of the specific transform, see the two "TODO:" lines in transforms.qmd

Copyright and Licensing

Please list the copyright holder for the work you are submitting (this will be you or your assignee, such as a university or company):
Simons Foundation

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC BY-ND 4.0 (https://creativecommons.org/licenses/by-nd/4.0/)

[spinkney]

I'll try and take a look this week

[bob-carpenter]

I can take a shot at this, @spinkney. I think the definition is clear enough.

[bob-carpenter]

I have most of this written up and just have to double check. I should be able to push the doc tomorrow (Wednesday). I'm just transating from the code. It's really nice that there's no Jacobian adjustment.

[bob-carpenter]

I wrote out the math for the transform. It'd be great if @spinkney could review, though I'm pretty sure it at least matches the code as is (perhaps modulo a +1 or -1 on indexing, but I triple-checked all that, too).