Documentation: Add runtimeconfig to next.config.js options index

[lixiaowei11111]

Summary

In the next.config.js Configuration documentation, the index of all available options does not list the runtimeconfiguration.

However, the runtimeoption (for setting Edge or Node.js) is actively used and documented within the Middleware documentation.

I propose adding the runtimeoption to the main configuration index with a brief description and a link to its detailed usage in the Middleware docs. This would significantly improve its discoverability for developers looking for all available build-time configuration options.

Additional information

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Example

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