Provide guidance to subprojects for how to prepare VPAT for their technology

The [Voluntary Product Accessibility Template](https://www.itic.org/policy/accessibility/vpat) (VPAT) is a standard way for technology products to declare their accessibility for others to reference. This is important both for users and also for institutions that are assessing the accessibility status of tools they consider using.

Subprojects likely don't have the expertise or capacity to know that this exists, or to know how to prepare one. It might be helpful to provide some guidelines to Jupyter subprojects, and guidelines for the value of these documents and realistic ways to prepare them.

### Context / why is this important

Accessibility has always been important in Jupyter, and there's a lot we can still do to improve. It is getting even more important as institutions begin to adopt more formal and restrictive policy about what technology they are able to use.

I suspect that this is going to become a bigger and bigger issue as time goes on, and this is a way for us to think about and get ahead of the problem before we are _forced_ to do so (e.g., because of an exodus of educational institutions due to the lack of documents like these). I don't know the timeline for this, but I do see it coming sometime...

Start by reading the linked ITIC VPAT reference and reviewing the issue's context about Jupyter subprojects. Produce guidance explaining the value of VPATs and realistic ways subprojects can prepare them; done means the guidance is usable by subprojects without assuming accessibility expertise.

Written by the indexing model from the issue text.