Add Gitea Actions workflows, CI config, and docs

.gitea/ISSUE_TEMPLATE/config.yml

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+# Issue template chooser configuration
+# See: https://docs.gitea.com/usage/issue-pull-request-templates
+
+blank_issues_enabled: true
+
+contact_links:
+  - name: Security Vulnerability
+    url: https://git.hiddenden.cafe
+    about: >
+      DO NOT open a public issue for security vulnerabilities.
+      Please email security@hiddenden.cafe or see SECURITY.md for
+      responsible disclosure instructions.