volo-build README uses obsolete Builder API and config schema

## Bug Report

### Version

`cloudwego/volo` `main` at `58a9eebc4941eb2090c8a07ea142bf073f3527c9` (`volo-build` 0.12.3).

### Platform

Windows 11 x86_64, Rust/Cargo 1.95.0. The problem is platform-independent.

### Crates

`volo-build`

### Description

The direct-usage example in `volo-build/README.md` no longer matches the public API or configuration model.

The documented build script:

```rust
fn main() {
volo_build::Builder::default().write().unwrap();
}
```

fails in a minimal consumer with:

```text
error[E0599]: no function or associated item named `default` found for struct `volo_build::Builder`
```

The same README also shows a top-level `idls:` list, while the current configuration model requires named `entries`, each with a protocol and `services`.

I expected the README example to compile and its YAML to deserialize with the current `volo-build` release. Replacing the call with `volo_build::ConfigBuilder::default().write()` and using the current `entries` schema makes the minimal consumer compile successfully with `cargo check --offline`.

Open volo-build/README.md and compare the direct-usage build script and YAML example with the current volo-build 0.12.3 API and configuration model. Update the example to use ConfigBuilder and the entries schema with protocols and services, then verify a minimal consumer with cargo check --offline and confirm the YAML deserializes.

Written by the indexing model from the issue text.