No bash completion (or missing instructions) on Mac (Big Sur)

**Describe the bug**
A clean Homebrew install of Azure CLI used to include instructions for modifying `.bash_profile` or `.zshrc` to enable bash completion. This no longer appears to be the case.

**To Reproduce**
Install Homebrew
`brew install zsh`
`brew install azure-cli`

No information provided on how to enable auto completions.

**Expected behavior**

Include helper command like `az completion zsh` that can be added, or provide instructions for how to properly enable completions.

After much digging I have determined this should be the following for ZSH:
```
autoload bashcompinit && bashcompinit
source $(brew --prefix)/etc/bash_completion.d/az
```

Note that `brew --prefix` most likely corresponds to `/opt/homebrew`

**Environment summary**

Mac OS 11.2.3. -- M1 Mac (ARM64)
zsh 5.8 (arm-apple-darwin20.2.0)

```
brew --version
Homebrew 3.1.3
Homebrew/homebrew-core (git revision 7beae5ab57; last commit 2021-04-22)
Homebrew/homebrew-cask (git revision d0318692c3; last commit 2021-04-22)
```

```
az --version
azure-cli 2.22.1

core 2.22.1
telemetry 1.0.6

Extensions:
azure-cli-ml 1.25.0
subscription 0.1.4
account 0.2.1

Python location '/opt/homebrew/Cellar/azure-cli/2.22.1/libexec/bin/python'
Extensions directory '/Users/bverst/.azure/cliextensions'

Python (Darwin) 3.8.9 (default, Apr 2 2021, 17:46:14)
[Clang 12.0.0 (clang-1200.0.32.29)]

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```