enhance: Add comprehensive .gitignore for deb-mock project

- Add mock-specific build artifacts (chroot/, mock-*, mockroot/)
- Include package build files (*.deb, *.changes, *.buildinfo)
- Add development tools (.coverage, .pytest_cache, .tox)
- Include system files (.DS_Store, Thumbs.db, ._*)
- Add temporary and backup files (*.tmp, *.bak, *.backup)
- Include local configuration overrides (config.local.yaml, .env.local)
- Add test artifacts and documentation builds
- Comprehensive coverage for Python build system project

This ensures build artifacts, chroot environments, and development
tools are properly ignored in version control.

docs/script/cibuild

@@ -0,0 +1,9 @@
+#!/bin/sh
+
+set -e
+
+bundle exec jekyll build
+bundle exec htmlproofer ./_site --check-html --check-sri
+bundle exec rubocop -D --config .rubocop.yml
+bundle exec script/validate-html
+gem build jekyll-theme-slate.gemspec