[DOCS] Various documentation issues with the civo_instance documentation

[fernando-villalba]

Description

Issue

The Civo Instance documentation has various issues (some of this apply to all our documentation, other tickets will be created for those later):

1. There is an example with civo_instances_size. This data source does not exist, remove this reference or create the data source so it makes sense
2. We need to change the section called "Read-Only" to "Attributes Reference", this is how it is most commonly referred to everywhere. Calling it Read-Only also makes sense but it is peculiar. Also, we need to change the "Schema" section name to "Argument Reference". Example of how these categories are often named can be found here.
3. This example, as any other example we give should be fully functional

Screenshots

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Additional information

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