Add CI release workflow, CHANGELOG.md, issue templates

- .github/workflows/release.yml: on tag push, build exe via PyInstaller,
  package portable zip, attach SHA-256, create GitHub Release.
- CHANGELOG.md: Keep a Changelog format, semver.
- .github/ISSUE_TEMPLATE/: bug_report.yml + feature_request.yml + config.yml
  routing security reports to private advisories.

.github/ISSUE_TEMPLATE/config.yml

@@ -0,0 +1,5 @@
+blank_issues_enabled: false
+contact_links:
+  - name: Security vulnerability
+    url: https://github.com/Engelgardt23/dhcpsrv/security/advisories/new
+    about: Please report security issues privately via GitHub Security Advisories — not as a public issue.