docs: release notes for the v12.2.6 release
docs: release notes for the v12.2.6 release

crisbeto/material2,angular/components,angular/components,josephperrott/material2,angular/components,angular/components,crisbeto/material2,josephperrott/material2,josephperrott/material2,crisbeto/material2,josephperrott/material2,crisbeto/material2,josephperrott/material2