docs: add CONTRIBUTING.md and issue/PR templates (#13)

[koteshyelamati]

Completes the remaining community-health files from #13. SECURITY.md already landed in #16; this PR adds the rest of the acceptance criteria:

• CONTRIBUTING.md — environment setup (Rust toolchain + Soroban CLI), workspace layout including the campaign/ canonical-contract rule, the make build/test/fmt/lint/audit/deny workflow, Conventional Commits, the PR process, and a security section that points to SECURITY.md.
• .github/ISSUE_TEMPLATE/bug_report.md — structured bug report (affected component, repro steps, environment).
• .github/ISSUE_TEMPLATE/feature_request.md — feature request template.
• .github/PULL_REQUEST_TEMPLATE.md — PR checklist mirroring the local make checks and Conventional Commits.

All content is derived from the existing README and Makefile so the instructions match the repo's actual tooling. This is a docs-only change — no code paths are touched.

[Alqku]

Hi @koteshyelamiti 👋 — great additions to the community-health files! Docs don't require CI so that's not a blocker here. However I can't find a referenced issue on this PR. Could you link the issue (or have a maintainer open/assign one to you) so I can confirm the contributor-matches policy? Once that's sorted I'm happy to merge 🙏

[GBOYEE]

I'd like to work on this.

Approach:

• I'll audit the affected code paths and patch the vulnerability
• Verify with existing tests + add new ones if needed

Estimated effort: ~1-2 hours. PR incoming shortly.

[Alqku]

Thanks for the docs work! Issue #13 doesn't have an assignee yet — please ping a maintainer to get it assigned to you before we can land this. Also worth a double-check on the two referenced template paths vs. the repo layout. 🙏

[koteshyelamati]

Thanks @Alqku! I'll reach out to a maintainer to get #13 assigned to me and update here once that's done. I also re-checked the template paths against the repo's .github/ layout — bug_report.md, feature_request.md, and PULL_REQUEST_TEMPLATE.md are all in the right place. Let me know if anything else is needed before merge.