docs(volo-build): update README config example

[dajiaohuang]

Motivation

volo-build/README.md still uses the removed Builder::default() API and a pre-refactor top-level idls configuration. Following the direct-use instructions therefore fails to compile and the shown YAML does not match the current configuration model.

Fixes: #668

Solution

• Use ConfigBuilder::default() in the build.rs example.
• Replace the obsolete YAML with current named entries for Thrift and Protobuf, including the required repository lock field.
• Link to the complete configuration format guide and fix the surrounding grammar.

Validation

• Reproduced the old snippet's E0599 compiler error in a minimal path-dependent consumer.
• Verified the corrected ConfigBuilder consumer with cargo check --offline.
• Deserialized the README's exact YAML block as volo_build::model::SingleConfig and asserted both entries.
• cargo test -p volo-build --locked (16 passed).
• cargo rustdoc -p volo-build --all-features --locked -- --deny warnings.
• cargo fmt --all -- --check.
• git diff --check.