Make the item deletion rule much more targeted

What does this MR do and why?

In !3205 (merged), we removed our ability to process any Markdown files that start with an underscore (_).

Whether or not we have nanoc process _index.md files (per: gitlab-org/technical-writing-group/gitlab-docs-hugo#55 (closed)), we certainly can't with the current rule as it is, and we should probably be more targeted in our item deletions anyway.

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