[DOCS] Extending Salt Cloud - outdated example of module that is using libcloud

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Description
An example to follow for writing a cloud driver module based on libcloud is the Linode module which is not using libcloud since 2015:
Move Linode driver to use REST API instead of libcloud

Suggested Fix
Add example of cloud driver module that is using libcloud

Type of documentation
Salt documentation

Location or format of documentation
WRITING CLOUD DRIVER MODULES

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