build: add .gitignore file with common exclusions

Add standard .gitignore entries to exclude development environment files,
OS-generated files, temporary files, and other common patterns from version control

.gitignore

@@ -0,0 +1,25 @@
+# ElectrumX data directory
+/data/
+
+# Docker volumes and logs
+*.log
+
+# IDE and editor files
+.vscode/
+.idea/
+*.swp
+*.swo
+*~
+
+# OS generated files
+.DS_Store
+.DS_Store?
+._*
+.Spotlight-V100
+.Trashes
+ehthumbs.db
+Thumbs.db
+
+# Temporary files
+*.tmp
+*.temp