docs: add community standards

[snkmcb]

Summary

• Add a Code of Conduct, contributor guide, and security policy.
• Add bug-report and feature-request issue templates.
• Add issue-template routing and a pull request checklist.
• Link the community standards from the root README.

Validation

• VS Code diagnostics report no errors for the changed documents.
• Markdown relative-link and trailing-whitespace checks passed.
• CMake/CTest were not run because this is a documentation-only change.