fix: broken relative link due to trailing slash

- use full relative path instead of docusaurus controlled intro.md path
works in both cases (with/without trailing slash)

Author: songlim327

docs/quick-start/sales/sales-invoice/intro.md

@@ -19,4 +19,4 @@ dataEntryList = {[
 ]}
 />
 
-[Click to learn more about E-Invoice](../e-invoice)
+[Click to learn more about E-Invoice](../../e-invoice/intro.md)