docs: add configuration architecture section to config-center README

[shenshichao163-oss]

What this PR does

Adds a Configuration Architecture section to script/config-center/README.md to clarify the relationship between the two configuration files (application.yml and config.txt) and how they relate to each other.

Changes

Added a new section before "Script Introduction" that includes:

1. Explanation of the two configuration files:

   • application.yml — Server-side Spring Boot configuration (port, logging, config/registry/store type)
   • config.txt — Client-side transaction configuration (transport, vgroup mapping, store settings)

2. Architecture diagram showing the config flow:

   • Server reads application.yml → connects to Config Center
   • Config Center stores config.txt content (pushed by scripts)
   • Client reads from Config Center

3. FAQ section answering:

   • Where should seata.config.nacos.data-id be placed?
   • What does config.txt do?
   • Should I use config.txt or application.yml?

Issue Link

Fixes #5608

Motivation

Issue #5608 reports confusion about Seata configuration: which file goes where, what each config item means, and how application.yml and config.txt relate to each other. This PR adds clear documentation directly in the config-center README to address these questions.

Checklist

• I have searched the issues of this repository and believe this is not a duplicate.
• I have checked that this modification does not cause CI to fail.
• I will sign the Apache CLA if required.