add vignette on Schnute/Richards growth?

[iantaylor-NOAA]

@arni-magnusson created an excellent vignette describing how the Schnute/Richards growth function is implemented in SS3 which I think would be helpful to add to the website. Discussion of the vignette is in https://github.com/nmfs-ost/ss3-doc/pull/265 and the files themselves are in a "placeholder repository" at https://github.com/arni-magnusson/ss3-richards.

A few questions and tasks:

• [ ] is the SS3 website the right place to put this?
• [ ] if so, should the model files that the vignette depends on be stored in this repo as well to keep things reproducible, or just post the resulting PDF and maybe attached the files to this issue as a way to track down the source if needed in the future

Not specific to this vignette,

• [ ] consider creating a page that lists all the vignettes with brief descriptions which might be easier to look over than the vignettes pulldown menu on the home page

[Rick-Methot-NOAA]

I support adding this to the website's vignettes, and to including a zip with model input files.