Point docs at git.fejl40.nu after the move from GitHub

- README / CONTRIBUTING: clone from the Gitea repo.
- SECURITY: Gitea has no private security advisories; report to
  support@fejl40.nu instead.
- RELEASING: release URL on Gitea, plus a note that build.yml still uses
  the GitHub-only action-gh-release step, so no releases are produced on
  Gitea yet (server self-update via git is unaffected).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CONTRIBUTING.md

@@ -21,8 +21,8 @@ very welcome.
 
 ```bash
 # Clone and set up a virtual environment
-git clone https://github.com/your-org/gdpr-scanner.git
-cd gdpr-scanner
+git clone https://git.fejl40.nu/Fejl40.nu/GDPRScanner.git
+cd GDPRScanner
 python3 -m venv venv
 source venv/bin/activate          # macOS / Linux
 venv\Scripts\activate             # Windows