Fix example woodworking tree, make links relative to repo

[majabojarska]

I was browsing this project and stumbled upon the broken link/image for the woodworking example skill tree:

This PR:

• fixes the issue described above.
• replaces the raw GitHub user content paths with repo-relative paths.
  • The CDN paths are not guaranteed to stay the same over time.
  • This approach allows the maintainers to see image changes immediately in the local README.md preview, when modifying the images and PDFs.
• The example images now also link to their corresponding PDF files. This is debatable, but in my opinion it's best to redirect the user to a vector-based PDF, instead of the rasterized PNG preview.

[sjpiper145]

Thanks for taking the time to fix this one up @majabojarska, much appreciated!!