Validate and document using of php private static extensions

[emteknetnz]

There are two ways to define Silverstripe config

• yaml
• private static $extensions

Based on this comment

extension config that is inherently necessary for a class to function as expected should be in the class itself.
e.g. SiteTree is always expected to be versioned, so we have the versioned extension declared in the class directly. This implies those extensions should never be removed - whereas extensions added via yaml should be allowed to be removed without negative impacts.

We should:

• Validate the any existing non-optional config is declared in php extensions and composer requirements if necessary
• Update documentation that makes it clear when to use each type of config