Feature: Show line numbers

## Description
Show line numbers in the number to guide author and readers.

## Impact
It improves document readability and user experience. It helps authors to understand how many lines they have written and readers to resume the reading or to share content with others.

The issue names no file, entry point, or test. Begin by locating the editor rendering and document-layout code, then check existing tests for displayed content. Before implementation, clarify which editor surfaces need line numbers and what formatting or interaction is expected; done should include agreed behavior and coverage for it.

Written by the indexing model from the issue text.