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.

[Copilot]

Pull request overview

Adds a new “Configuration Architecture” section to script/config-center/README.md to clarify how Seata server/client configuration relates to the config-center scripts (addressing confusion raised in #5608).

Changes:

• Documents the roles of application.yml vs script/config-center/config.txt
• Adds an ASCII diagram for the configuration flow through a config center
• Adds a short FAQ (including seata.config.nacos.data-id placement)

Suppressed comments (3)

script/config-center/README.md:51

• script/config-center/config.txt is described as a "client-side" config file, but it contains both client and server keys (e.g. store.* / store.mode) and is not read directly at runtime. It’s a template the scripts publish into the config center (for Nacos, each line is published as its own DataId).

2. **`script/config-center/config.txt`** — The **client-side** configuration file (key-value properties format). It contains:
   - Transport settings (protocol, heartbeat, thread pool)
   - Transaction group mappings (`service.vgroupMapping.xxx`)
   - Client-side settings (lock, undo, retry policies)
   - Storage configuration (when `store.mode=db` or `redis`)

script/config-center/README.md:94

• The answer currently says seata.config.nacos.data-id should be in the client only and suggests the server otherwise reads a local config.txt. In practice, data-id applies to any Seata process (server or client) using Nacos config, and config.txt is just the script input template (not a runtime local config file).

**Q: Where should `seata.config.nacos.data-id` be placed?**

A: In the **client's** `application.yml`, this setting tells the Seata client which Nacos data ID to read for client configuration. On the server side, `seata.config.type=nacos` in `application.yml` tells the server to read from Nacos instead of the local `config.txt`.

script/config-center/README.md:75

• The diagram implies the config center stores a single "config.txt" document, but the Nacos/Apollo/etc scripts actually publish key/value entries (for Nacos, each config key becomes a DataId). Renaming this box makes the flow match what the scripts do.

│  │  config.txt (client config)   │                          │
│  │  - transport.*                │                          │
│  │  - service.vgroupMapping.*    │                          │
│  │  - store.* (db settings)      │                          │
│  └────────────────────────────────┘                          │

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[WangzJi]

Thanks for working on this. The new section is helpful, but a couple of configuration roles need to be corrected:

1. seata.config.nacos.data-id is not client-only. The server also needs it when using an all-in-one Nacos configuration such as seataServer.properties.
2. config.txt is not a client-side configuration file. It is the source used by the publishing scripts and contains shared, client-only, and server-only settings.
3. The server does not read a local config.txt at runtime, and using both files is not always required.

Could you update the description and diagram to reflect these distinctions?