Add documentation for the roadm impairment feature #286

Job	Run time
Tests on paywalled platforms	0s
Tox test (py38)	2m 35s
Tests on other platforms (windows-2019, 3.10)	3m 12s
Tox test (py39)	2m 32s
Tests on other platforms (windows-2022, 3.11)	3m 37s
Tox test (py310)	2m 34s
Tests on other platforms (windows-2022, 3.12)	3m 35s
Tox test (py311)	2m 24s
Tests on other platforms (macos-12, 3.11)	2m 36s
Tox test (py312-cover)	5m 10s
Tests on other platforms (macos-13, 3.12)	3m 24s
Tox test (docs, graphviz)	1m 54s
PyPI packaging	0s
Docker image	0s
	33m 33s

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