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/Release-Notes-1.2.15.md

@@ -0,0 +1,14 @@
+---
+layout: default
+title: Release Notes 1.2.15
+---
+
+mock-1.2.15 introduce these changes:
+
+* Fedora 24 chroot configs (and Fedora 21 was removed).
+* ccache plugin is by default off - to copy behaviour of Koji and Copr.
+* ~/.config/mock.cfg is parsed too (beside ~/.mock/user.cfg).
+
+And several bugfixes:
+* buildroot is removed as root [RHBZ#1294979](http://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=1294979)
+* "local" dnf plugin is disable [RHBZ#1264215](http://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=1264215)