doc-gen: migrate scalar functions (string) documentation 1/4 #36431

Job	Run time
Check License Header	2m 9s
linux build test	3m 47s
cargo test (macos-aarch64)	13m 17s
Check cargo fmt	40s
build with wasm-pack	3m 13s
Verify MSRV (Min Supported Rust Version)	9m 35s
Verify Vendored Code	54s
cargo check datafusion	4m 49s
cargo test (amd64)	11m 29s
cargo check no default features	7m 49s
cargo check functions	1m 31s
cargo doc	4m 31s
cargo test datafusion-cli (amd64)	4m 50s
cargo test doc (amd64)	16m 39s
cargo examples (amd64)	14m 54s
Run sqllogictest with Postgres runner	4m 12s
cargo test pyarrow (amd64)	3m 2s
verify benchmark results (amd64)	8m 20s
check Cargo.toml formatting	2m 52s
clippy	7m 11s
check configs.md and ***_functions.md is up-to-date	3m 51s
cargo test hash collisions (amd64)	15m 45s
	2h 25m 20s

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