Document forge verify-contract --show-standard-json-input option (#956)

I found this option in this issue - foundry-rs/foundry#3507

This was helpful for verifying a contract on Zora's blockscout based explorer.

I don't really know where this standard comes from or anything about it, so hoping someone more knowledgeable is willing to take this over the finish line.

src/reference/forge/forge-verify-contract.md

@@ -68,6 +68,9 @@ you can specify a file containing **space-separated** constructor arguments with
 
 {{#include ../common/retry-options.md}}
 
+`--show-standard-json-input`  
+    Command outputs JSON suitable for saving to a file and uploading to block explorers for verification.
+
 `--watch`  
     Wait for verification result after submission.  
     Automatically runs `forge verify-check` until the verification either fails or succeeds.