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/de/installation.md

@@ -0,0 +1,57 @@
+# BaraDB — Installation
+
+## Voraussetzungen
+
+- **Nim >= 2.2.0** (`curl https://nim-lang.org/choosenim/init.sh -sSf | sh`)
+- **Git**
+- **OpenSSL** (für TLS)
+
+## Aus dem Quellcode bauen
+
+```bash
+git clone https://github.com/katehonz/barabaDB.git
+cd barabadb
+nimble build_release
+```
+
+Die Binärdateien werden im `build/` Verzeichnis erstellt:
+- `build/baradadb` — Datenbank-Server (TCP + HTTP)
+- `build/baramcp` — MCP Server für AI-Agenten
+
+## Debug-Build
+
+```bash
+nimble build_debug
+```
+
+## Tests ausführen
+
+```bash
+nimble test
+```
+
+## Docker
+
+```bash
+docker compose up -d
+```
+
+## Verifizierung
+
+```bash
+./build/baradadb --version
+# BaraDB v1.1.2 — Multimodal Database Engine
+
+./build/baramcp --data-dir ./data &
+echo '{"jsonrpc":"2.0","id":1,"method":"initialize","params":{}}' | ./build/baramcp
+```
+
+## Manuelle Kompilierung
+
+```bash
+# Server
+nim c -d:release --opt:speed -o:build/baradadb src/baradadb.nim
+
+# MCP Server
+nim c -d:release --opt:speed -o:build/baramcp src/baramcp.nim
+```