refactor(writer): move Hugo index-file rename from Writer.Write into DocumentWorker.ProcessNode

[VelmiraS]

What this PR does / why we need it:
Moves the Hugo _index.md rename decision from Writer.Write into DocumentWorker.ProcessNode, so the Writer interface is no longer aware of Hugo-specific logic. No behavior change for users.

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:
The FakeWriter counterfeiter was updated manually (removed arg5 []string). TestWriteHugoIndexRename moved from fswriter_test to document_worker_rename_test.

Release note:

Writer.Write signature drops the IndexFileNames parameter; callers that implement the Writer interface must update their signature.

[gardener-prow]

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