Add comprehensive documentation with i18n support (EN/BG)

- Add docs/ folder with English (en/) and Bulgarian (bg/) documentation
- Create index.md with language switching and links
- English docs: installation, quickstart, architecture, baraql, storage,
  schema, lsm, btree, vector, graph, fts, columnar, transactions,
  distributed, protocol, udf, api-binary, api-http, api-websocket
- Bulgarian docs: installation, quickstart, architecture, baraql,
  schema, lsm, btree, vector, graph, fts, transactions, distributed
- Update README license to BSD 3-Clause
- Add LICENSE file with BSD 3-Clause text

docs/en/transactions.md

@@ -0,0 +1,61 @@
+# Transactions & MVCC
+
+MVCC (Multi-Version Concurrency Control) with snapshot isolation and deadlock detection.
+
+## Usage
+
+```nim
+import barabadb/core/mvcc
+
+var tm = newTxnManager()
+let txn = tm.beginTxn()
+
+# Write operations
+discard tm.write(txn, "key1", cast[seq[byte]]("value1"))
+discard tm.write(txn, "key2", cast[seq[byte]]("value2"))
+
+# Savepoint
+tm.savepoint(txn)
+discard tm.write(txn, "key3", cast[seq[byte]]("value3"))
+discard tm.rollbackToSavepoint(txn)  # undo key3
+
+# Commit
+discard tm.commit(txn)
+```
+
+## Transaction Isolation
+
+BaraDB uses **snapshot isolation**:
+- Readers don't block writers
+- Writers don't block readers
+- Each transaction sees a consistent snapshot
+
+## Deadlock Detection
+
+```nim
+import barabadb/core/deadlock
+
+var detector = newDeadlockDetector()
+if detector.detectCycle(txn1, txn2):
+  echo "Deadlock detected!"
+```
+
+## Write-Ahead Log
+
+```nim
+import barabadb/storage/wal
+
+var wal = newWAL("./wal")
+wal.append(txnId, "SET key value")
+wal.flush()
+```
+
+## Savepoints
+
+Nested transaction savepoints:
+
+```nim
+tm.savepoint(txn, "sp1")
+# ... operations ...
+tm.rollbackToSavepoint(txn, "sp1")
+```