chore: make plugin publish-safe for PyPI

Remove private git.eskimo.dev URLs (README install -> `pip install
certbot-dns-yeil`; setup.py url -> docs.yeil.app/dns). Update README to the
api.yeil.app/v1/dns gateway + gateway-relative paths. Flesh out setup.py
metadata (long_description from README, classifiers, python_requires,
project_urls, keywords). Add an MIT LICENSE file and a Python .gitignore
(so build/ dist/ *.egg-info/ __pycache__ stay out of the repo).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitignore

@@ -0,0 +1,15 @@
+# Python
+__pycache__/
+*.py[cod]
+*$py.class
+
+# Packaging / build artifacts
+build/
+dist/
+*.egg-info/
+.eggs/
+
+# Virtualenvs
+.venv/
+venv/
+env/