[APIServer][Docs] Add user guide for retry behavior & configuration

apiserversdk/docs/retry-behavior.md

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+# KubeRay APIServer Retry Behavior
+
+The KubeRay APIServer automatically retries failed requests to the Kubernetes API when transient errors occur.
+This built-in mechanism uses exponential backoff to improve reliability without requiring manual intervention.
+As of `v1.5.0`, the retry configuration is hard-coded and cannot be customized.
+This guide describes the default retry behavior.
+
+## Default Retry Behavior
+
+The KubeRay APIServer automatically retries with exponential backoff for these HTTP status codes:
+
+- 408 (Request Timeout)
+- 429 (Too Many Requests)
+- 500 (Internal Server Error)
+- 502 (Bad Gateway)
+- 503 (Service Unavailable)
+- 504 (Gateway Timeout)
+
+Note that non-retryable errors (4xx except 408/429) fail immediately without retries.
+
+The following default configuration explains how retry works:
+
+- **MaxRetry**: 3 retries (4 total attempts including the initial one)
+- **InitBackoff**: 500ms (initial wait time)
+- **BackoffFactor**: 2.0 (exponential multiplier)
+- **MaxBackoff**: 10s (maximum wait time between retries)
+- **OverallTimeout**: 30s (total timeout for all attempts)
+
+which means $$\text{Backoff}_i = \min(\text{InitBackoff} \times \text{BackoffFactor}^i, \text{MaxBackoff})$$
+
+where $i$ is the attempt number (starting from 0).
+The retries will stop if the total time exceeds the `OverallTimeout`.