Document `link_name` in Guppy decorator API docs

[CalMacCQ]

with guppylang v0.21.10 added the ability to pass a link_name argument when defining structs, functions, enums etc. We should update the manually written decorator docs to reflect this.

[maximilianruesch]

This involves a question around how much discussion of the compilation product (the HUGR) happens in the docs themselves. Perhaps once libraries rolls around, link names can offer a very short explanation and otherwise point to that?

[CalMacCQ]

Sounds reasonable. I'd be interested to know what your plans are for documenting Guppy libraries.

[maximilianruesch]

In essence, something like Quantinuum/guppylang#1552. Location, look and feel not yet finalised.