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

[camilamacedo86]

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:
#4827 (review)

Extra Labels

No response

[camilamacedo86]

c/c @mabulgu