doc-gen: migrate scalar functions (string) documentation 3/4 #36362

Job	Run time
linux build test	3m 44s
Check License Header	2m 11s
build with wasm-pack	3m 13s
cargo test (macos-aarch64)	11m 9s
Verify Vendored Code	49s
Check cargo fmt	44s
Verify MSRV (Min Supported Rust Version)	9m 26s
cargo check no default features	7m 51s
cargo check datafusion	4m 43s
cargo check functions	1m 35s
cargo test (amd64)	11m 18s
cargo test datafusion-cli (amd64)	4m 45s
check Cargo.toml formatting	2m 59s
clippy	7m 14s
check configs.md and ***_functions.md is up-to-date	3m 45s
cargo test hash collisions (amd64)	15m 33s
cargo doc	5m 11s
cargo test doc (amd64)	18m 38s
cargo examples (amd64)	14m 46s
Run sqllogictest with Postgres runner	4m 18s
cargo test pyarrow (amd64)	3m 6s
verify benchmark results (amd64)	8m 1s
	2h 24m 59s

Provide feedback