Readme: fix broken links in Studio README.md

# Problem The [Supabase Studio README](https://github.com/supabase/supabase/tree/master/apps/studio) currently contains several broken or outdated references: - The Contributing section mentions a checklist that links to a non-existent file (404): https://github.com/supabase/supabase/blob/master/apps/studio/contributing/contributing-checklists.md - The Developer Quickstart notes reference several dead links, including: https://github.com/supabase/infrastructure/blob/develop/docs/contributing.md These dead links can create confusion and slow down onboarding for contributors. ## Describe the improvement Restore or create the missing contributing checklist file in apps/studio/contributing/ and link to it correctly in the README. Update or remove dead links in the Developer Quickstart section (internal, infrastructure, repo).