Building on Windows is broken due to symbolic link

[Jeeyong Um]

Is there an existing issue?

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• This is not a support question.

Description of bug

#3087 added the symbolic link pointing to docs tosubstrate/primitives/runtime, but on Windows (tested on Windows 10), symbolic link cannot be recognized, so the following line causes file not found error during compiling Substrate.

polkadot-sdk/substrate/primitives/runtime/src/generic/unchecked_extrinsic.rs

Line 60 in 89604da

 #[cfg_attr(feature = "std", doc = simple_mermaid::mermaid!("../../docs/mermaid/extrinsics.mmd"))] 

Steps to reproduce

Compile sp-runtime on Windows.

[Bastian Köcher]

We don't really support Windows. However, if you have a fix, we are happy to accept your pr that fixes the issue.