docs: linear retry strategy

[yaythomas]

Document the linear retry strategy across all three SDKs. The SDKs ship a linear builder alongside exponential, and docs/sdk-reference/error-handling/retries.md only covers exponential.

• TypeScript: createLinearRetryStrategy
  • feat(sdk): add linear retry strategy with configurable increment aws-durable-execution-sdk-js#513
  • feat(sdk): align linear retry strategy with exponential helper aws-durable-execution-sdk-js#652
• Python: create_linear_retry_strategy and linear/fixed presets
  • feat: expand RetryPresets with fixed, linear, and slow strategies aws-durable-execution-sdk-python#335
  • feat(sdk): align create_linear_retry_strategy with create_retry_strategy (parity with JS #652) aws-durable-execution-sdk-python#484
• Java: RetryStrategies.linear
  • feat: add linear retry strategy aws-durable-execution-sdk-java#468

Related: #155 (withRetry helper).