docs: add changelog for PR 13229 (support ${cert_pem} authn placeholder)

changes/ce/feat-13229.en.md

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+Introduced the `${cert_pem}` placeholder within authentication parameters.
+
+This allows for the transmission of the full certificate in PEM format (excluding headers and footers) to external authenticators,
+such as HTTP authenticators, when clients are connected to the EMQX SSL listener.