CAPI docs on using multiple machinedeployments

[frezbo]

Some simple docs on adding different Serverclasses to a could be really helpful

Scenario:

• User has two different serverclasses matching different machine architectures
• How do add an extra machine to the same cluster referencing the second serverclass

[bzub]

Here's an example of using multiple serverclasses and machinedeployments. In case it's useful. https://github.com/bzub/kpt-pkgs/tree/main/examples/cluster-api-workload-sidero