Document how to use CAPI providers

[bsctl]

Add a new section in the official guide documenting how to use supported CAPI providers with Kamaji.

[bhavyastar]

Hey @bsctl , I would like to work on this issue.

[prometherion]

Thanks @bhavyastar!

Feel free to reach out on the Slack channel if you have any questions, especially considering the Cluster API's Control Plane contract.

[prometherion]

Documentation is crucial for building more trust and knowledge around the project, at the same time knowing all the internals and CAPI quirks is pretty challenging: @bhavyastar let me know how we can help you, otherwise, feel free to withdraw your effort so somebody else can take over.