Re-add section about extension API to `CONTRIBUTING.md` (#59702)

This was lost in https://github.com/zed-industries/zed/pull/58627

Release Notes:

- N/A
3601a7c8c2Finn Evers committed on 6/22/2026, 12:55:51 PM· committed by GitHubparente1bfff8
1 file changedLine totals unavailable