Add community infrastructure: issue + PR templates, CoC

Gitea issue templates (.gitea/issue_template/):
- bug.yml: structured form requiring version, environment,
  what-happened, repro steps, expected, optional logs. Routes
  security reports to security@librete.ch instead of public
  issues.
- feature.yml: prompts for the underlying problem before the
  proposed solution, plus alternatives and out-of-scope.

Pull request template (.gitea/PULL_REQUEST_TEMPLATE.md):
checklist for tests, lint, manual exercise, docs, and changelog.
Asks for explicit reviewer notes so trade-offs surface in the
PR description rather than being lost in chat.

CODE_OF_CONDUCT.md: links to Contributor Covenant 2.1 verbatim
rather than inlining; documents scope, reporting address
(conduct@librete.ch), and points enforcement at the Covenant's
own Enforcement Guidelines.

README links the docs/ tree, CONTRIBUTING, and the CoC so new
contributors find the entry points.

Closes #31.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

README.md

@@ -76,6 +76,19 @@ by Alon Swartz, used and redistributed under the MIT License. See
 [NOTICE](./NOTICE) for the upstream commit hash at fork time and
 instructions for tracking upstream changes.
 
+## Documentation
+
+- [User guide](./docs/user-guide.md)
+- [Self-hosting](./docs/self-hosting.md)
+- [API reference](./docs/api.md)
+- [Operations](./docs/operations.md)
+
+## Contributing
+
+See [CONTRIBUTING.md](./CONTRIBUTING.md) for setup, conventions, and
+the PR process. By participating you agree to the
+[Code of Conduct](./CODE_OF_CONDUCT.md).
+
 ## License
 
 MIT — see [LICENSE](./LICENSE). Copyright is shared between the original