Document Neovim tab mark settings (Refs #429)

[devin-ai-integration]

Summary

Adds a Neovim troubleshooting note explaining that visual tab/indent marks are editor decorations, not Markdown Oxide output. The note points users to :set nolist, vim.opt.list = false, or indent-guide plugin configuration.

Refs #429

Review & Testing Checklist for Human

• Confirm the guidance is accurate for the Neovim tab/indent marks shown in help / ideas to remove tab marks #429.
• Confirm this note belongs in the Neovim setup section and is worded clearly for users.
• Optional: verify in Neovim that :set nolist hides built-in whitespace markers.

Notes

Validation run locally:

• cargo fmt --check
• cargo clippy
• cargo test

Link to Devin session: https://app.devin.ai/sessions/2c747aa5ec774e73811ae5b3c1f3522d
Requested by: @Feel-ix-343

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✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.