Publish "Detailed error explanations" design documentation page (#3780)

The page was added back in #3734, but was not added to the index, so

https://smithy-lang.github.io/smithy-rs/design/client/detailed_error_explanations.html,
which is exposed here:


https://github.com/smithy-lang/smithy-rs/blob/10a92052b1ca4f6442b15f544dd2940f613bd008/rust-runtime/aws-smithy-runtime-api/src/client/connection.rs#L24-L27

currently points to a 404.

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