Add Security Headers section to deployment guide

- Added section 3.7 with all 6 HTTP security headers
- Includes HSTS, CSP, X-Frame-Options, X-Content-Type-Options, Referrer-Policy, Permissions-Policy
- Testing instructions and online validation tools
- Links to detailed SECURITY_HEADERS.md documentation
- Fixed .gitignore to properly allow INFO/Deployment/*.md files

.gitignore

@@ -44,7 +44,8 @@ gitea.pub
 !themes/**/scripts/
 telegram/
 migration-s3/
-INFO/
+INFO/*
+!INFO/Deployment/
 !INFO/Deployment/*.md
 forms/.env