Document absence of special function-like components

[Tamschi]

Asteracea component syntax is quite concise and Rust erases zero-sized instances at compile time. Add such, components without instance fields will behave in a function-like way.

Additionally, it is possible to return a Node created by a fragment! invocation from a plain Rust function, and to use this function in Rust expressions in other Asteracea macro calls.

The documentation should include an assert that the instance size actually is 0.

[Tamschi]

This issue is part of #16.

[Tamschi]

The relevant assert has been implemented as test in fb04d72, but it would be good to show off a component that prints its own size to that end.

[Tamschi]

Implemented by 60fff02 and available at https://schichler.dev/Asteracea/static_components/ex_stateless_components_are_zero-sized.html.