docs: link contributor onboarding pathway

[sapunyangkut]

Summary

• add the new-contributor onboarding learning pathway to the README's contribution resources
• use the same training.galaxyproject.org domain as the surrounding links

Closes #6961.

Validation

• confirmed the learning-pathway URL returns HTTP 200
• confirmed open-PR searches for 6961 and GitHub Copilot return no matches
• git diff --check
• staged diff contains one file and two added lines only

Automation disclosure

This documentation-only change was prepared and tested with OpenAI Codex automation. It received advisory AI reviews from an independent local agent and ChatGPT Web; both returned approval. No human review is claimed.

[bgruening]

Thanks @sapunyangkut very nice

@melpetera is that what you had in mind as well?

[melpetera]

Hi,
This indeed corresponds to the idea we mentionned, thanks!
To note, there is also a CONTRIBUTING.md file in the repository, so an alternative could be to mention this file in the readme, and to add the link to the learning pathway in the CONTRIBUTING.md directly maybe?