build(aio): move doc-gen stuff from angular.io (#14097)

docs/content/cheatsheet/class-decorators.md

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+@cheatsheetSection
+Class decorators
+@cheatsheetIndex 5
+@description
+{@target ts}`import { Directive, ... } from '@angular/core';`{@endtarget}
+{@target js}Available from the `ng.core` namespace{@endtarget}
+
+@cheatsheetItem
+syntax(ts):
+`@Component({...})
+class MyComponent() {}`|`@Component({...})`
+syntax(js):
+`var MyComponent = ng.core.Component({...}).Class({...})`|`ng.core.Component({...})`
+description:
+Declares that a class is a component and provides metadata about the component.
+
+@cheatsheetItem
+syntax(ts):
+`@Directive({...})
+class MyDirective() {}`|`@Directive({...})`
+syntax(js):
+`var MyDirective = ng.core.Directive({...}).Class({...})`|`ng.core.Directive({...})`
+description:
+Declares that a class is a directive and provides metadata about the directive.
+
+@cheatsheetItem
+syntax(ts):
+`@Pipe({...})
+class MyPipe() {}`|`@Pipe({...})`
+syntax(js):
+`var MyPipe = ng.core.Pipe({...}).Class({...})`|`ng.core.Pipe({...})`
+description:
+Declares that a class is a pipe and provides metadata about the pipe.
+
+@cheatsheetItem
+syntax(ts):
+`@Injectable()
+class MyService() {}`|`@Injectable()`
+syntax(js):
+`var OtherService = ng.core.Class(
+    {constructor: function() { }});
+var MyService = ng.core.Class(
+    {constructor: [OtherService, function(otherService) { }]});`|`var MyService = ng.core.Class({constructor: [OtherService, function(otherService) { }]});`
+description:
+{@target ts}Declares that a class has dependencies that should be injected into the constructor when the dependency injector is creating an instance of this class.
+{@endtarget}
+{@target js}
+Declares a service to inject into a class by providing an array with the services, with the final item being the function to receive the injected services.
+{@endtarget}