Docs: Demonstrate non-enum internal types in example (#3314)

* Docs: Demonstrate non-enum internal types in example

Previously example only demonstrated internal enumeration type. 
To show that it works for other internal types the same way the example was updated with an additional struct Pet::Attributes type.

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci



Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>