Transactions.md redirect (#545)

https://nomicon.io/DataStructures/Transaction is blank. I'm adding a
link to the Transactions spec page.

We also could delete this page but I'm choosing not to because:
1. There might be links to this page that would break.
2. Many chains consider a Transaction to be a core data structure - so
this blank page is the first place I looked to learn about the NEAR tx
format. I think having a link here that points people in the right
direction could be nice for people new to NEAR.

specs/DataStructures/Transaction.md

@@ -1 +1,3 @@
 # Transaction
+
+See [Transactions](../RuntimeSpec/Transactions.md) documentation in the [Runtime Specification](../RuntimeSpec) section.