Link mermaid docs to crate that uses them (paritytech#3087)

While including files cross crate works locally. When pushed to
crates.io each crate is seperate so the file path no longer exists.

Instead change it to a symlink, which cargo will change to a read
directory when published.

substrate/primitives/runtime/docs

@@ -0,0 +1 @@
+../../../docs

substrate/primitives/runtime/src/generic/unchecked_extrinsic.rs

@@ -57,7 +57,7 @@ type UncheckedSignaturePayload<Address, Signature, Extra> = (Address, Signature,
 /// could in principle be any other interaction. Transactions are either signed or unsigned. A
 /// sensible transaction pool should ensure that only transactions that are worthwhile are
 /// considered for block-building.
-#[cfg_attr(feature = "std", doc = simple_mermaid::mermaid!("../../../../../docs/mermaid/extrinsics.mmd"))]
+#[cfg_attr(feature = "std", doc = simple_mermaid::mermaid!("../../docs/mermaid/extrinsics.mmd"))]
 /// This type is by no means enforced within Substrate, but given its genericness, it is highly
 /// likely that for most use-cases it will suffice. Thus, the encoding of this type will dictate
 /// exactly what bytes should be sent to a runtime to transact with it.