docs: add German (DE) documentation + update all docs for Sessions 10-12

New German documentation (docs/de/):
- index.md, quickstart.md, installation.md
- baraql.md, graph.md, vector.md, mcp.md

Updated English documentation:
- changelog.md: all Sessions 10-12 features
- graph.md: SQL GRAPH_TABLE, CREATE GRAPH, all 8 algorithms, Cypher, similarity_nodes, node2vec
- vector.md: hybrid RAG, chunk(), embed_text(), auto-embed, nl_to_sql(), schema_prompt()
- baraql.md: new AI & Cross-Modal Functions section, updated keyword tables
- mcp.md: MCP Server documentation (new file)
- index.md: added German (DE) language link

docs/index.md

@@ -5,12 +5,13 @@
 ## Documentation Languages
 
 - [English](en/)
-- [Български (Bulgarian)](bg/)
-- [Русский (Russian)](ru/)
-- [فارسی (Farsi)](fa/)
-- [中文 (Chinese)](zh/)
-- [Türkçe (Turkish)](tr/)
 - [العربية (Arabic)](ar/)
+- [Български (Bulgarian)](bg/)
+- [Deutsch (German)](de/)
+- [فارسی (Farsi)](fa/)
+- [Русский (Russian)](ru/)
+- [Türkçe (Turkish)](tr/)
+- [中文 (Chinese)](zh/)
 
 ---