Explain that dynamic refs can implement generics

[handrews]

Fixes #3601.

Includes an example. This intentionally does not explain how dynamic referencing works, as there are better resources available in both the spec and (more readably) the official JSON Schema blog.

[handrews]

To add a little more context (with apologies for the rapid-fire comment spam):

• This feature is (as noted above) highly desired and has been requested repeatedly since 2015
• It is implemented in some places (I believe that @gregsdennis , who wrote the JSON Schema Blog's post about it, supports it in C#)
• $dynamicRef and $dynamicAnchor are kind of the discriminator of 3.1 – there was a long period where I noticed many OpenAPI tools claimed they supported everything, but with an asterisk saying "except discriminator... part of the reason for this is that these new keywords are moderately complex. But part of it is that people don't understand that they can support a common programming paradigm that is otherwise unsupportable. We will not see these implemented unless they are connected to real customer use cases, and that's what this PR does.

[mikekistler]

Looks good! 👍