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-5.1.1.md

@@ -0,0 +1,29 @@
+---
+layout: default
+title: Release Notes - Mock 5.1.1
+---
+
+Released on 2023-09-18.
+
+## Mock 5.1.1 bugfixes
+
+- [commit#1e13b56ce3c0efdf81][] caused "basedir" to be created only once per Mock
+  run, but likewise directory "rootdir" was created only once.
+
+  Since Mock automatically unmounts rootdir **after each build** and then
+  also **removes the rootdir** directory to finish the cleanup tasks (at
+  least if tmpfs or other "root" plugin is in use, --resultdir is in
+  use, ...), subsequent builds failed to re-mount the rootdir with, e.g.:
+
+      ERROR: Command failed:
+      $ mount -n -t tmpfs -o mode=0755 -o nr_inodes=0 -o size=140g mock_chroot_tmpfs /var/lib/mock/fedora-37-x86_64-1694797505.326095/root
+
+  This caused problems e.g. [in Fedora Copr][copr_issue#2916] where each
+  Mock build is actually a two-step build done like:
+
+      mock --spec foo.spec --sources . --resultdir ...
+
+  So Mock first builds SRPM, and then builds RPMs (two builds in one run).
+
+[commit#1e13b56ce3c0efdf81]: https://github.com/rpm-software-management/mock/commit/1e13b56ce3c0efdf81
+[copr_issue#2916]: https://github.com/fedora-copr/copr/issues/2916