feat: enhance Postman integration with nested folder support and improved documentation handling

[Statsly-org]

Summary

Refresh README.md to match current behavior: path-based nested Postman folders (pathFolderNesting), short request titles via x-pman-name / x-name vs. long summary in Docs, and how sync state + merge removes both new path keys and legacy folder layouts from older releases.

Motivation

Users upgrading or switching from tags / flat path grouping to nested folders need a single place that explains the problem, the options, and what happens on merge—without spelunking the source.

What changed

• Added a "Pain points" section (titles vs. summaries, folder browsing, stale folders after strategy changes or upgrades).
• Documented folderStrategy, pathFolderNesting (nested vs flat), and folderPathStripPrefix with a concrete example (/auth/user/admin/create → Auth → User → Admin).
• Expanded merge / managedFolders and legacy removal behavior.
• Tightened the usage example and the local smoke-test steps (playground + nested demo route).

Test plan

• N/A (documentation only). Prior npm test and npm run lint were green for code changes in this branch; confirm README renders on GitHub (anchors, tables).

Issue: Closes #8