feat(ledgers): explain that this is called a "block" in other blockch…

…ains (#195)

I found Stellar's non-standard use of the word "ledger" pretty surprising. I think it's useful to explain the concept in terms that other blockchain users might be familiar with.

docs/fundamentals-and-concepts/stellar-data-structures/ledgers.mdx

@@ -5,6 +5,8 @@ sidebar_position: 10
 
 A ledger represents the state of the Stellar network at a point in time. It is shared across all Core nodes in the network and contains the list of accounts and balances, orders on the distributed exchange, and any other persisting data.
 
+Other blockchains refer to this concept as a "block", and the entire blockchain as "the ledger".
+
 In every Stellar Consensus Protocol round, the network reaches consensus on which transaction set to apply to the last closed ledger, and when the new set is applied, a new “last closed ledger” is defined. Each ledger is cryptographically linked to the unique previous ledger, creating a historical chain that goes back to the genesis ledger.
 
 Data is stored on the ledger as ledger entries. Possible ledger entries include: