chore(docs): create missing markdown files for cdk items

src/cdk/a11y/focus-key-manager.md

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+### FocusKeyManager
+`ListKeyManager` manages the focus of an item based on keyboard interaction.

src/cdk/coercion/coercion.md

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+### Coercion
+
+Utility functions for coercing `@Input`s into specific types.

src/cdk/collections/collections.md

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+### Collections
+
+A set of utilities for managing collections.

src/cdk/keycodes/keycodes.md

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+### KeyCodes
+
+Commonly used keycode constants.
+
+#### Example
+```ts
+import {Directive} from '@angular/core';
+import {UP_ARROW, DOWN_ARROW, LEFT_ARROW, RIGHT_ARROW} from '@angular/cdk/keycodes';
+
+@Directive({
+  selector: '[count-arrows]'
+  host: {
+    (keypress): 'handleKeyPress($event)'
+  }
+})
+export class ArrowCounterDirective {
+  arrowPressCount = 0;
+
+  handleKeyPress(event: KeyboardEvent) {
+    if ([UP_ARROW, DOWN_ARROW, LEFT_ARROW, RIGHT_ARROW].includes(event.keyCode)) {
+      this.arrowPresscount++;
+    }
+  }
+}
+```
+

src/cdk/observers/observers.md

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+### Observers
+
+A directive for observing when the content of the host element changes. An event is emitted when a
+mutation to the content is observed.
+
+#### Example
+
+```html
+<div class="projected-content-wrapper" (cdkObserveContent)="projectContentChanged()">
+  <ng-content></ng-content>
+</div>
+```

src/cdk/overlay/overlay.md

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+### Overlay
+
+A service used to manage overlays.

src/cdk/platform/platform.md

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+### Platform
+
+A service for determing the current platform.

src/cdk/rxjs/rxjs.md

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+### RxJS Usage
+When dealing with RxJS operators it is important to be aware that using the "patch" imports will
+pollute the global Observable object (e.g. `import "rxjs/add/operator/map"`).  When creating a
+component library, it is highly recommended to import the operator functions directly (e.g.
+`import "rxjs/operator/map"`):
+
+```ts
+// NO
+import 'rxjs/add/operator/map';
+someObservable.map(...).subscribe(...);
+
+// YES
+import {map} from 'rxjs/operator/map';
+map.call(someObservable, ...).subscribe(...);
+```
+
+#### RxChain
+Because this approach can be inflexible when dealing with long chains of operators. You should use
+the `RxChain` class to help with it:
+
+```ts
+// Before
+someObservable.filter(...).map(...).do(...);
+
+// After
+RxChain.from(someObservable).call(filter, ...).call(map, ...).subscribe(...);
+```

src/cdk/scrolling/scrolling.md

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+### Scrolling
+
+Some things to help with scrollling.

src/cdk/scrolling/viewport.md

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+### Viewport
+
+A utility for getting the bounds of the browser viewport.