Document using secret references for static token authentication

[markphelps]

Description

Add documentation explaining how to use secret references to securely manage static authentication tokens in Flipt v2. This feature should already work but isn't documented. We also need to test it to verify

Context

Currently, static tokens are documented as being defined directly in configuration files:

• https://docs.flipt.io/v2/configuration/authentication#static-token

However, Flipt v2 supports secret references that can be used throughout the configuration:

• https://docs.flipt.io/v2/configuration/overview#secret-references
• https://docs.flipt.io/v2/configuration/secrets

Users should be able to use secret references for static token credentials to avoid hardcoding sensitive tokens in configuration files.

Documentation Updates Needed

1. Update Static Token Documentation

Update the Static Token authentication section to include examples using secret references.

Current example:

authentication:
  methods:
    token:
      enabled: true
      tokens:
        - credential: "some-plaintext-token"
          metadata:
            name: "Development Token"

Add example with secret references:

authentication:
  methods:
    token:
      enabled: true
      tokens:
        - credential: "{{ secrets.vault.flipt/tokens/ci-token.value }}"
          metadata:
            name: "CI Pipeline Token"
            description: "Token for automated deployments"
        - credential: "{{ secrets.file.tokens/dev-token.value }}"
          metadata:
            name: "Development Token"

2. Cross-Reference Related Documentation

Ensure proper cross-references between:

• Secret References
• Secrets Configuration
• Static Token Authentication

Related Issues

• Feature: Integrate Secrets Management as Backend for Token and Session Storage flipt#4560 - Feature request for enhanced token management with secrets