Minor: consolidate ConfigExtension example into API docs #36476

Job	Run time
Check cargo fmt	44s
Check License Header	2m 22s
linux build test	3m 48s
build with wasm-pack	3m 13s
cargo test (macos-aarch64)	11m 17s
Verify MSRV (Min Supported Rust Version)	9m 19s
Verify Vendored Code	49s
cargo test (amd64)	13m 25s
cargo check datafusion	4m 47s
cargo check functions	1m 31s
cargo check no default features	7m 56s
cargo examples (amd64)	15m 6s
check Cargo.toml formatting	3m 3s
clippy	7m 14s
check configs.md and ***_functions.md is up-to-date	4m 26s
cargo test hash collisions (amd64)	17m 41s
cargo doc	4m 31s
cargo test doc (amd64)	18m 28s
Run sqllogictest with Postgres runner	4m 50s
verify benchmark results (amd64)	8m 8s
cargo test pyarrow (amd64)	3m 39s
cargo test datafusion-cli (amd64)	5m 16s
	2h 31m 33s

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