docs: RDS IAM Authentication

[brendanshanahan]

Summary

This PR adds documentation for the AWS RDS IAM database authentication feature introduced in open-webui/open-webui (PR linked below).

Changes:

• docs/getting-started/advanced-topics/rds-iam-authentication.md
  • Covers prerequisites (enabling IAM auth on RDS, IAM policy, GRANT rds_iam TO , SSL CA bundle), environment variable configuration, Docker Compose and Kubernetes examples, token refresh behavior, and a troubleshooting section.
• docs/reference/env-configuration.mdx
  • Adds entries for the two new environment variables introduced by the linked PR:
    • DATABASE_ENABLE_IAM_TOKEN_AUTH — enables IAM token auth in place of a static password
    • DATABASE_CA_PATH — path to the RDS CA certificate bundle for SSL verification

Implementation PR: open-webui/open-webui#23580

Related discussion: open-webui/open-webui#20783

[pr-validator-bot]

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