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/bg/transactions.md

@@ -0,0 +1,27 @@
+# Транзакции & MVCC
+
+Multi-Version Concurrency Control със snapshot изолация.
+
+## Употреба
+
+```nim
+import barabadb/core/mvcc
+
+var tm = newTxnManager()
+let txn = tm.beginTxn()
+
+discard tm.write(txn, "key1", cast[seq[byte]]("value1"))
+discard tm.write(txn, "key2", cast[seq[byte]]("value2"))
+
+tm.savepoint(txn)
+discard tm.rollbackToSavepoint(txn)
+
+discard tm.commit(txn)
+```
+
+## Изолация
+
+BaraDB използва **snapshot isolation**:
+- Читателите не блокират писатели
+- Писателите не блокират читатели
+- Всяка транзакция вижда консистентен моментна снимка