doc: add `severity` example #840

Job	Run time
Miri	3m 4s
Check fmt & build docs	13s
Build & Test (fancy, 1.70.0, ubuntu-latest)	46s
Build & Test (fancy, 1.70.0, macOS-latest)	1m 5s
Minimal versions check (ubuntu-latest)	18s
Build & Test (fancy, 1.70.0, windows-latest)	1m 18s
Minimal versions check (macOS-latest)	30s
Build & Test (fancy, stable, ubuntu-latest)	38s
Minimal versions check (windows-latest)	31s
Build & Test (fancy, stable, macOS-latest)	59s
Build & Test (fancy, stable, windows-latest)	2m 27s
Build & Test (syntect-highlighter, stable, ubuntu-latest)	55s
Build & Test (syntect-highlighter, stable, macOS-latest)	1m 57s
Build & Test (syntect-highlighter, stable, windows-latest)	2m 57s
	17m 38s

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