Document type aliases

[ss2165]

Guppy v1 adds language-level type aliases. We should document the syntax, intended use cases, and current limitations in the language guide.

Relevant PR:

• feat!: Add type aliases guppylang#1645

Suggested scope:

• Add a language guide section for type aliases.
• Explain how aliases interact with generics and existing type syntax.
• Include examples using aliases in function signatures.
• Mention any restrictions or unsupported alias forms.
• Ensure API/reference coverage via Update API reference for Guppylang v1 #157.