Add conceptual docs on accessing HttpContext #770

Job	Run time
build (ubuntu-latest, Debug)	3m 29s
build (windows-latest, Debug)	8m 37s
build (macos-latest, Release)	2m 56s
build (macos-latest, Debug)	3m 40s
build (ubuntu-latest, Release)	3m 52s
build (windows-latest, Release)	5m 20s
publish-coverage	0s
	27m 54s

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