docs(security): document custom app token headers

[AkashKumar7902]

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Description

Documents the custom app API token header support for HTTP and gRPC callbacks. This covers the DAPR_APP_API_TOKEN_HEADER environment variable, the dapr.io/app-token-header Kubernetes annotation, replacement and default behavior, and the case where a custom name is configured without APP_API_TOKEN.

Companion runtime: dapr/dapr#10239.

Validation: npm run build:production completed successfully with Hugo v0.147.8.

Issue reference

Related to dapr/dapr#7147.