CODE_OF_CONDUCT.md linked from CONTRIBUTING.md does not exist

[jordigilh]

Summary

CONTRIBUTING.md requires all participants to follow the CNCF Code of
Conduct and links to a local CODE_OF_CONDUCT.md, but that file does not
exist in the repo.

(Two sibling docs originally flagged in the same audit pass --
docs/gateway-api-support.md and docs/troubleshooting.md, both linked
from README.md -- have since been added and are no longer broken; only
CODE_OF_CONDUCT.md remains missing.)

Evidence

CONTRIBUTING.md:111-120 at upstream/main
(5bda5841a64d5d6a6d46062f0c0169e20c7b7b59):

## Code of Conduct

All participants must follow the
[CNCF Code of Conduct][coc].

[conventions]: docs/conventions.md
[dco]: https://developercertificate.org/
[issues]: https://github.com/praxis-proxy/operator/issues
[disc]: https://github.com/orgs/praxis-proxy/discussions
[coc]: CODE_OF_CONDUCT.md

CODE_OF_CONDUCT.md is absent from the repository root (and from the tree
entirely).

Risk

Low. It's a broken link at the exact point where the project tells
contributors what conduct standard they're agreeing to -- more of a
"first impression" paper cut than a functional problem, but worth fixing
given the project explicitly requires adherence to it.

Suggested fix

Add a CODE_OF_CONDUCT.md at the repo root. Since the project already
states it follows the CNCF Code of Conduct, the simplest fix is the
standard CNCF boilerplate file that just points to the org-wide CoC (as
used by most CNCF sandbox/incubating projects), rather than drafting a new
one from scratch.

Severity: Low