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:
https://github.com/kubernetes-sigs/kubebuilder/pull/4827#pullrequestreview-2866897777

Extra Labels

No response

[camilamacedo86]

c/c @mabulgu

[rayowang]

@camilamacedo86 Hi, I just happen to have some Java-related tasks recently, so I’ll take care of this. I’d like to ask, where should this document be placed?

[rayowang]

/assign

[mabulgu]

Hi @rayowang first if you like lets implement the solution. I will take cwre of it shortly. Then we will be able to have a doc for it so you can work on this issue. Does it make sense?

[nerdeveloper]

/assign