Document function and callable types

[ss2165]

Guppy v1 changes the function/callable model with function item types, a Callable protocol, the Callable type rename to Function, and higher-order functions with unitary modifiers.

Relevant PRs:

• feat!: Add function item type guppylang#1950
• feat!: Add Callable protocol guppylang#1929
• feat!: Rename Callable type guppylang#1860
• feat!: Higher order functions with unitary modifiers guppylang#1768
• Related modifier/protocol work: feat!: Turn modifiers into protocols guppylang#1951

Suggested scope:

• Own the general "functions as values / callable constraints" explanation here.
• Cross-link to protocol docs in Language guide section on Guppy protocols #121 for protocol mechanics.
• Cross-link to modifier docs in Language guide page on modifiers (control, power, dagger) #98 and the QPE example in Port cannonical quantum phase estimation example to use Guppy modifiers #122 for control/dagger usage.
• Explain how callable/function types are written in v1 and how they differ from pre-v1 docs.
• Include at least one higher-order function example with unitary modifiers.
• Add API/reference coverage via Update API reference for Guppylang v1 #157.