Document how to update arrays and objects in Ember Octane

## Background

- [Learning Team meeting notes from August 20, 2020](https://github.com/emberjs/core-notes/blob/master/learning-team/2020-08/2020-08-20.md#ember-arrays-vs-native-arrays)
- [A Discord conversation](https://discordapp.com/channels/480462759797063690/480777444203429888/741663906245771294)

## Description

We currently don't describe well how to update tracked arrays and objects in Ember Octane. On Discord, now and then, a developer does ask how they can do so.

```javascript
@tracked myArray = [];
@tracked myObject = {};

@action updateMyArray() {
// How?
}
@action updateMyObject() {
// How?
}
```

- [In-Depth Topics - Autotracking In-Depth - Plain Old JavaScript Objects (POJOs)](https://guides.emberjs.com/release/in-depth-topics/autotracking-in-depth/#toc_plain-old-javascript-objects-pojos)
- [In-Depth Topics - Autotracking In-Depth - Arrays](https://guides.emberjs.com/release/in-depth-topics/autotracking-in-depth/#toc_arrays)
- [Upgrading - Tracked Properties - Plain Old JavaScript Objects (POJOs)](https://guides.emberjs.com/release/upgrading/current-edition/tracked-properties/#toc_plain-old-javascript-objects-pojos)
- [Upgrading - Tracked Properties - Arrays](https://guides.emberjs.com/release/upgrading/current-edition/tracked-properties/#toc_arrays)

## Possible TODOs

### 1. Arrays

- [ ] Create an example (the example may need to depend on the surrounding page context)
- [ ] Explain that Ember does not react to `this.myArray.push(...);`.
- [ ] Explain possible solutions
- immutable approach
- `EmberArray`
- `tracked-built-ins`

### 2. Objects

- [ ] Create an example (the example may need to depend on the surrounding page context)
- [ ] Explain that Ember does not react to `this.myObject.myProperty = ...;`.
- [ ] Explain possible solutions
- immutable approach
- native class
- `tracked-built-ins`

### 3. Backport

Once the changes are approved for `release` directory, backport the changes all the way back to Ember 3.15.

## Questions

- We have 2 pages (Autotracking In-Depth & Tracked Properties) with almost similar content. Can one page refer to another?