Add docs: how to create custom markers for unsupported file extensions

[camilamacedo86]

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What do you want to happen?

We should add a documentation page that shows how to use Kubebuilder as a library to define your own marker support for file types not handled by core (like .rs, .tpl, .java, etc.).

This is useful when:

• You're building an external plugin
• You want to scaffold files that are not Go
• Your file extensions aren't (and shouldn't be) part of the core commentsByExt map

The doc should show:

• How to create your own Marker using machinery.Marker
• How to plug that into your scaffolding logic

See this example with .rs:
https://github.com/kubernetes-sigs/kubebuilder/pull/4827#pullrequestreview-2866897777

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