docs: add Turkish README translation (README_tr.md) (#13750)

## Summary
Add a complete Turkish translation of the README and include a Turkish
language badge across all existing README files.

## Changes
- **New file**: `README_tr.md` - Full Turkish translation of README.md,
covering all sections (What is RAGFlow, Demo, Latest Updates, Key
Features, System Architecture, Get Started, Configurations, Docker
Image, Development from Source, Documentation, Roadmap, Community,
Contributing)
- **Updated 9 existing README files** (README.md, README_zh.md,
README_tzh.md, README_ja.md, README_ko.md, README_id.md,
README_pt_br.md, README_fr.md, README_ar.md) to include the Turkish
language badge in the language selector

## Impact
- 10 files changed, 417 insertions
- Follows the same structure and conventions as other language-specific
README files (README_ja.md, README_ko.md, etc.)
- Turkish badge uses the same styling pattern (highlighted with DBEDFA
in README_tr.md, standard DFE0E5 in others)

---------

Co-authored-by: bakiburakogun <bakiburakogun@users.noreply.github.com>

README.md

@@ -14,6 +14,7 @@
   <a href="./README_pt_br.md"><img alt="Português(Brasil)" src="https://img.shields.io/badge/Português(Brasil)-DFE0E5"></a>
   <a href="./README_fr.md"><img alt="README en Français" src="https://img.shields.io/badge/Français-DFE0E5"></a>
   <a href="./README_ar.md"><img alt="README in Arabic" src="https://img.shields.io/badge/Arabic-DFE0E5"></a>
+  <a href="./README_tr.md"><img alt="Türkçe README" src="https://img.shields.io/badge/Türkçe-DFE0E5"></a>
 </p>
 
 <p align="center">