[Doc] Add doc for installing Azure CLI with pip

There are platforms or architectures where we don't officially distribute native packages:

- https://github.com/Azure/azure-cli/issues/7368
- https://github.com/Azure/azure-cli/issues/18766
- https://github.com/Azure/azure-cli/issues/19591
- https://github.com/Azure/azure-cli/issues/16526

The solution is to install `azure-cli` from PyPI: https://pypi.org/project/azure-cli/

```
pip install azure-cli
```

We do have the doc https://docs.microsoft.com/en-us/cli/azure/install-azure-cli-linux?pivots=script which is internally using `pip` to install `azure-cli`. It calls https://github.com/Azure/azure-cli/blob/dev/scripts/curl_install_pypi/install which then calls https://github.com/Azure/azure-cli/blob/dev/scripts/curl_install_pypi/install.py.

However, this method is

1. Too complex/automated and leaves users with very few customization options
2. Interactive, which is not suitable for automated installation: 
3. Not actively maintained. For example, it support Ubuntu up to 18.04: https://github.com/Azure/azure-cli/blob/50ccb24528eb36529d1432808f3ff385cc4c04aa/scripts/curl_install_pypi/install.py#L366-L367