Add "Available since version" to function reference

[WardBrian]

Submission Checklist

• Builds locally
• Declare copyright holder and open-source license: see below

Summary

This closes #417. Each function has had a version number added to it.

This represents the minimum version one should expect the function to appear in in its current form, meaning renames or changes in return type/signature count as moving up a version.

This was built from a list automatically generated from old documentation back to version 2.0, with some manual adjustments provided by @rok-cesnovar, @bob-carpenter, and by looking through old release notes. I'm fairly confident most of the version numbers are correct, and when they miss they miss newer, so we never say something was available during a version it wasn't, we just may be late here or there.

Here is how it looks in PDF:

And web:

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 4.0 (https://creativecommons.org/licenses/by/4.0/)

[bob-carpenter]

A question, some optional fixes depending on your patience, and a lot of gratitude for finding and fixing so many existing errors. This doesn't necessarily need any fixes, but I'm still marking as requesting changes to wait for answers to the questions before merging.

[bob-carpenter]

I'm afraid merging the complex return types caused a conflict. It should be good to go after that.