docs: add multi-language documentation (ru, fa, zh, tr, ar)

docs/zh/fts.md

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+# 全文搜索引擎
+
+具有 BM25 和 TF-IDF 排名的倒排索引。
+
+## 用法
+
+```nim
+import barabadb/fts/engine
+
+var idx = newInvertedIndex()
+idx.addDocument(1, "Nim is a fast programming language")
+idx.addDocument(2, "Python is popular for data science")
+
+let results = idx.search("programming language")
+let fuzzy = idx.fuzzySearch("programing", maxDistance = 2)
+```
+
+## 排名方法
+
+### BM25
+
+```nim
+let bm25 = idx.searchBM25("query terms")
+```
+
+### TF-IDF
+
+```nim
+let tfidf = idx.searchTfidf("query terms")
+```
+
+## 搜索功能
+
+| 功能 | 描述 |
+|------|------|
+| Fuzzy search | 允许拼写错误（Levenshtein 距离） |
+| Wildcard | 前缀、后缀和中间通配符 |
+| Regex | 正则表达式模式 |
+| Phrase search | 精确短语匹配 |
+| Boolean | AND、OR、NOT 运算符 |
+
+## SQL 接口
+
+```sql
+CREATE INDEX idx_fts ON articles(body) USING FTS;
+SELECT * FROM articles WHERE body @@ 'machine learning';
+```