Support PEP 695 generics

[viccie30]

This builds on mkdocstrings/griffe#348 to document generic types, functions and type aliases.

The first commit makes a small change that the next commit builds upon. Because of the churn in tests, I've separated that commit.

I have added documentation, but not yet tests. This can probably only be merged once mkdocstrings/griffe#348 is merged.

[pawamoy]

I'll be working on this in the next few days, expect to see modifications coming in.

I wasn't fond of the new format_class and format_merged_init rendering utilities, because essentially what we want to do when merging the __init__ method into the class docs is to either:

• concatenate the type parameters of both the class and init method:

  class Foo[T][U](param1: T, param2: U)

  ...but that might prevent formatting by Black/Ruff if it's invalid syntax (I'm not sure it is)

• or actually merge/combine the type parameters together:

  class Foo[T, U](param1: T, param2: U)

  ...even if that's not entirely true, because as long as each type parameter cross-links to the right symbol, I think it's good.