Add list of classes with struct support

[katzuv]

It'll be nice to have a list of classes which support structs, and what values they hold. It looks like the only way to do this currently is to look through the source code.

[calcmogul]

What use case does this help with?

[katzuv]

First, just to let people know it exists. Second, for example we today needed to dig through the source code to find that Rotation2d has a struct, and to find out it holds the value in radians and not in degrees.

[calcmogul]

Yea, we should document the struct contents somewhere, because you need to know that to actually use it. Sounds like something that should be in API docs though rather than general docs.

[katzuv]

Should I open an issue in allwpilib?

[calcmogul]

Sure

[jasondaming]

Are we closing this issue then?