Add Core-Interfaces readme section on FluidObject

[anthony-murphy]

Really glad you found a good a way to accommodate proper delegation, yay! Can you update this doc comment to cover at least briefly about using Partial<IProvideFoo> if you want to do proper delegation? And I'm with Vlad - mentioning IProvideFoo.IFoo.IFoo doesn't necessarily bring clarity. That is indeed how the type magic works (it finds any prop that also exists on the prop's type), but not how this stuff is ever used.

Originally posted by @markfields in #8087 (comment)