DOC:Improve onboarding clarity and navigation in documentation

### What's wrong with the existing documentation

Hi maintainers 👋

I’ve set up the PictoPy documentation locally and explored the docs as a new contributor.

While going through the documentation, I noticed a few areas where onboarding and navigation could be improved for first-time users:
- A clearer “Getting Started” flow for new contributors
- Making the Setup section more prominent and easier to follow
- Adding a short high-level overview before detailed backend sections
- Better grouping or cross-linking of related documentation pages

I’d be happy to work on improving the documentation if this aligns with the project’s goals.
Please let me know your thoughts.

### Add ScreenShots

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