docs: in `sparse-v2.md`, use Markdown `warning` admonition #530

Job	Run time
Build binary artifacts (linux-x86_64-musl)	5m 8s
Build binary artifacts (linux-x86_64-gnu)	5m 19s
Build binary artifacts (linux-aarch64-musl)	5m 27s
Build binary artifacts (linux-aarch64-gnu)	5m 24s
Build binary artifacts (macos-x86_64)	7m 43s
Build binary artifacts (macos-aarch64)	3m 52s
Build binary artifacts (win-x86_64)	8m 43s
	41m 36s

Provide feedback