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/btree.md

@@ -0,0 +1,22 @@
+# B-Tree Индекс
+
+Подредена индексна структура за ефективни диапазонни заявки.
+
+## Употреба
+
+```nim
+import barabadb/storage/btree
+
+var btree = newBTreeIndex[string, string]()
+
+btree.insert("key1", "value1")
+let values = btree.get("key1")
+let range = btree.scan("key_a", "key_z")
+```
+
+## Функции
+
+- Подредени ключ-стойност двойки
+- Диапазонни заявки (BETWEEN, >, <, >=, <=)
+- Префиксни сканирания
+- Конфигурируем размер на страницата