Add Type Reference Documentation

[copperlight]

While working through the HLB tutorial, it took awhile for me to realize that the fs return type means filesystem.

As far as I can tell, there is no single page that lists all of the possible types that may exist in HLB with a short description of them.

It would be useful to have a page that provides an overview of all of the available types.

Maybe consider Python Built-in Types as an example, although it is likely more verbose than needed at first?

https://docs.python.org/3/library/stdtypes.html

[hinshun]

Yes definitely, I'll keep this in mind for our docs overhaul.