Add changelog posts for heading numbering and sidebar section reordering

[tommoor]

Summary

• Reviewed PRs merged in outline/outline over the past week (Aug 19–24) and picked out the two that stood out as customer-facing features worth a changelog post
• Add posts/heading-numbering.md covering the new per-document heading numbering display option (decimal, alphanumeric, and Harvard styles), shipped in outline/outline#13522
• Add posts/sidebar-sections.md covering the new ability to drag-reorder sidebar sections (Starred, Shared with me, Collections), shipped in outline/outline#13505

Both posts follow the frontmatter and tone of other recent single-feature posts (e.g. multi-select.md, profile-cards.md, mcp-improvements.md), with the markdown file name matching the slug field exactly.

Test plan

• Frontmatter fields (title, date, slug) match the shape expected by lib/posts.ts
• File names match slug exactly
• Manually compared tone/length against recent posts in posts/

---

Generated by Claude Code