Add approaches for `sum-of-multiples` exercise #2374

Job	Run time
Clippy	20s
configlet lint	6s
Check compilation (stable)	8s
markdown lint	6s
Check compilation (beta)	14s
Check exercises on nightly (benchmark enabled)	13s
Check Rust Formatting	27s
Run shellcheck on scripts	8s
Run repository tests	41s
	2m 23s

Provide feedback