Add more context to ecosystem/blueprint/testing/reference#updateconfig

Document:

• https://github.com/ton-org/sandbox?tab=readme-ov-file#updating-configuration
• https://github.com/ton-org/sandbox/blob/58448683d578e04a037d6f51a234c438ae012aa5/src/utils/config.ts#L3
• https://github.com/ton-org/sandbox/blob/58448683d578e04a037d6f51a234c438ae012aa5/src/utils/config.ts#L47