Add link to change.org petition in 2025-09-29-google-developer-registration-decree.md
Updates the blog post to have the "sign petitions" CTA at the end become a link to a pre-existing petition at https://www.change.org/p/stop-google-from-limiting-apk-file-usage
I would update the translations, but I'm not sure if I can just edit the .po directly (e.g., https://gitlab.com/marcprux/fdroid-website/-/blob/master/po/_posts.pt_BR.po?ref_type=heads#L23137), of if I need to go through Weblate.