docs: release notes for the v21.2.0-rc.1 release #2736

Job	Run time
build	27s
lint	2m 2s
build-e2e-windows	19s
e2e-windows	0s
publish-snapshots	0s
test	0s
Browser Compatibility Tests	0s
e2e	0s
e2e-snapshots	0s
e2e-package-managers	0s
	2m 48s

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