docs: add usage example for attr_list with def_list

[maruixin-kls]

Fixes #1123.

This PR adds a documentation example showing how to use the attr_list extension to assign IDs and classes to items within a def_list (Definition List).

I added this example to docs/extensions/attr_list.md to clarify the syntax for users combining these two extensions.

Description

This PR adds a documentation example showing how to use the attr_list extension to assign IDs and classes to items within a def_list (Definition List).

AI Assistance Disclosure

• No AI tools were used in preparing this PR.
• If AI tools were used, I have disclosed which ones, and fully reviewed and verified their output.

Checklist

• This PR follows the contribution guidelines.
• The code follows the Code Style Guide.
• The commit message follows the Commit Message Style Guide.
• I have added or updated relevant docs, including release notes if applicable which follow the [Documentation Style Guide](Documentation Style Guide).
• I have added or updated relevant tests.
• I have not requested, and will not request, an automated AI review for this PR.

[waylan]

This does not fix (resolve and close) #1123. It documents an existing workaround, but it does not add the ability to have IDs automatically generated.

[x] If AI tools were used, I have disclosed which ones, and fully reviewed and verified their output.

I don't see where you have disclosed which AI tools you used.