docs: fix stale contributing TODO and improve guide copy

[jacobzychowicz]

Summary

This PR makes a few small documentation fixes across the Paperback website repo.

Changes included:

• updated the stale TODO list in .github/CONTRIBUTING.md to reflect the repo's current state
• fixed contributor setup commands to use npm run dev, npm run build, and npm run preview
• corrected the website contribution guide link in src/contributing/index.md
• cleaned up a few wording and typo issues in public-facing docs

Why

Some of the contribution docs were outdated or misleading:

• the TODO list still referenced work that appears to already be completed
• the setup commands in the contributing guide did not match the scripts in package.json
• one contributing link pointed to the wrong section

These are small changes, but they make the docs more accurate for new contributors.