Add warning about AWS auth manager being in alpha stage in identity-c…

[imrichardwu]

Description

This PR updates the AWS IAM Identity Center setup documentation to clearly inform users that the AWS Auth Manager is currently alpha, experimental, and may change without notice.

This clarification helps prevent users from assuming the feature is stable when following the setup guide.

Changes

Documentation update

• Added a warning notice at the top of
  providers/amazon/docs/auth-manager/setup/identity-center.rst
• The notice explicitly states that the AWS Auth Manager is experimental and subject to change

Related issue

• closes: The AWS auth manager is alpha/experimental at the moment and may be subject to change without warning. #61720

[boring-cyborg]

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[boring-cyborg]

Awesome work, congrats on your first merged pull request! You are invited to check our Issue Tracker for additional contributions.