docs(exoflex): add docs for ActivityIndicator component (#74)

* add docs for ActivityIndicator component

* add preview image

* remove undefined from default props

packages/exoflex/docs/components/ActivityIndicator.md

@@ -0,0 +1,27 @@
+# ActivityIndicator
+
+Activity indicator is used to present progress of some activity in the app.
+
+### Preview
+
+![activity_indicator_preview](../assets/activity_indicator_preview.gif)
+
+### Props
+
+| Name               |              Type              |  Default  | Description                                                          |
+| ------------------ | :----------------------------: | :-------: | -------------------------------------------------------------------- |
+| `animating`        |           `boolean`            |  `true`   | Whether to show the indicator or hide it.                            |
+| `color`            |            `string`            |           | The color of the spinner. Will use the primary color from the theme. |
+| `size`             | `'small' \| 'large' \| number` | `'small'` | Size of the indicator.                                               |
+| `hidesWhenStopped` |           `boolean`            |  `true`   | Whether the indicator should hide when not animating.                |
+| `style`            |     `StyleProp<ViewStyle>`     |           | Additional style for the container.                                  |
+
+### Example
+
+```tsx
+<Provider>
+  <ActivityIndicator />
+
+  <ActivityIndicator size="large" color="salmon" />
+</Provider>
+```