docs(bidi): add initial guide (#5474)

guides/bidirectionality.md

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+# Angular Material bi-directionality
+
+### Setting a text-direction for your application
+The [`dir` attribute](https://developer.mozilla.org/en-US/docs/Web/HTML/Global_attributes/dir)
+is typically applied to `<html>` or `<body>` element of a page. However, it can be used on any
+element to apply a text-direction to a smaller subset of the page.
+
+All Angular Material components automatically reflect the LTR/RTL direction
+of their container. 
+
+### Reading the text-direction in your own components
+`@angular/cdk` provides a `Directionality` injectable that can be used by any component
+in your application. To consume this injectable, you must import `BidiModule`
+from `@angular/cdk`.
+
+`Directionality` provides two useful properties:
+* `value`: the current text direction, either `'ltr'` or `'rtl'`.
+* `change`: an `Observable` that emits whenever the text-direction changes. Note that this only
+captures changes from `dir` attributes _inside the Angular application context_. It will not
+emit for changes to `dir` on `<html>` and `<body>`, as these are assumed to be static.
+
+#### Example
+```ts
+@Component({ /* ... */})
+export class MyCustomComponent {
+  private dir: Direction;
+
+  constructor(directionality: Directionality) {
+    this.dir = directionality.value;
+    directionality.change.subscribe(() => {
+       this.dir = directionality.value;
+    });
+  }
+}
+```