Update extension docs to mention GNU GPLv3 is a valid license (#40933)

Merge after: 

- https://github.com/zed-industries/extensions/pull/3641

Release Notes:

- N/A
93136a9aaaJoseph T. Lyons committed on 10/22/2025, 7:59:39 PM· committed by GitHubparentf393138
1 file changedLine totals unavailable