Document and create diagrams to illustrate trait hierarchy

[ReeceHumphreys]

It seems that everyone is having a bit of a hard time keeping track of the relationships between many of our traits. It would be useful to update the readme with some explanations and mermaid diagrams.

[InsertCreativityHere]

The documentation for this already exists, just not on the docs site, since it's only recently come into existence;
see https://zeroc.atlassian.net/wiki/spaces/~austin/pages/1100742723/Slice+Compiler+Traits

I think this would be beyond the scope of the README.

[InsertCreativityHere]

And most importantly the UML diagram for them:

[InsertCreativityHere]

Once I get the compiler to a shippable state (hopefully soon), I plan to migrate all my documentation for the compiler from Confluence to the new docs site. It just hasn't been a high priority for me.

[InsertCreativityHere]

Also note that this diagram doesn't include the Member (for DataMember and Parameter) or Container traits.
Those are orthogonal to all the other traits, and self-explanatory enough I felt them not worth including in the diagram.