Add forgejo/SETUP_LOCAL.md documenting a self-contained local deployment
of Forgejo + ifcurl on Windows using Docker Desktop. Covers Nginx reverse
proxy for PR diff images, correct Forgejo v15 custom paths (/data/gitea),
libosmesa6 for headless rendering, and a git URL rewrite so the ifcurl
container can clone repos from localhost:3000 via the Docker-internal
forgejo hostname.
Add docker/Dockerfile for the ifcurl preview service.
Link SETUP_LOCAL.md from README.md and forgejo/README.md.
