[docs]: the page "Configuration sources" does not include environment configuration source

# Documentation request

The current implementation in`cloudinit.helpers.ConfigMerger._read_cfg()` also loads configuration from file specified in environment variable `CLOUD_CFG`:

https://github.com/canonical/cloud-init/blob/6725c0f896e0c55176dd0674668b527a3b2cbed9/cloudinit/helpers.py#L253

By the code, this environment configuration has higher priority than anything else but kernel parameters.

That is used in Microsoft Azure cloud data source. By default it [creates a single partition](https://github.com/canonical/cloud-init/blob/6725c0f896e0c55176dd0674668b527a3b2cbed9/cloudinit/sources/DataSourceAzure.py#L293-L302) on the Azure VM ephemeral disk. It also has higher priority than `cloud.cfg.d/` directory, so the only way to override it is supplying configuration via environment.
That is also reflected in the [Microsoft documentation (options 2 and 3)](https://learn.microsoft.com/en-us/troubleshoot/azure/virtual-machines/linux/create-swap-file-linux-vm#create-a-swap-partition).

FIX: document loading environment configuration in the list "Base configuration".

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*Reported from: https://cloudinit.readthedocs.io/en/latest/explanation/configuration.html*