Integrations documentation needs updating

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Setting up a new integration is difficult to get right, and our docs are no-longer accurate.

This was identified in !101771 (closed) when trying to add a new integration following the guide at https://docs.gitlab.com/ee/development/integrations/#rename-services-to-integrations

The following issues were identified:

• no mention of the need to define a title (this should be statically enforced with a method stub in Integration you must override)
• no mention of the need to define a self.to_param method (in most cases we can derive this from the class name)
• add more examples
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