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/Plugin-YumCache.md

@@ -0,0 +1,25 @@
+---
+layout: default
+title: Plugin YumCache
+---
+
+This plugin pre-mounts `/var/cache/{yum,dnf}` directories inside chroot, so the package manager's metadata don't have to be re-downloaded between subsequent mock commands (the caches survive `mock --clean` for example).  This plugin is needed because dnf (or yum) `--installroot DIRECTORY` commands store caches below the `DIRECTORY`.
+
+It mounts directories `/var/cache/mock/<chroot>/{dnf,yum}_cache` as `/var/cache/{dnf,yum}` in chroot.
+
+You can explicitly clean the package manager caches by `--scrub=dnf-cache` option.
+
+## Configuration
+
+This plugin is **enabled by default** and has the following values built-in:
+```python
+config_opts['plugin_conf']['yum_cache_enable'] = True
+config_opts['plugin_conf']['yum_cache_opts'] = {}
+config_opts['plugin_conf']['yum_cache_opts']['max_age_days'] = 30
+config_opts['plugin_conf']['yum_cache_opts']['max_metadata_age_days'] = 30
+config_opts['plugin_conf']['yum_cache_opts']['online'] = True
+```
+
+* `max_age_days` - when files in cache directory is older than this number of days, then such files are removed
+* `max_metadata_age_days` - when metadata (everything with suffix: ".sqlite", ".xml", ".bz2", ".gz") in cache directory is older than this number of days, then such files are removed.
+* `online` - when `False`, mock doesn't apply policies for `max_*_age_days` options (complements `--offline` option)