Update links to BAP with GitHub and update CONTRIBUTING.md

[erictleung]

Description

~This PR updates the book link for BAP because the original URL seems to be broken/unmaintained.~ This PR adds links to the GitHub code behind the BAP resource.

Moreover, as I was looking on how to contribute, I noticed that the CONTRIBUTING.md links were resolving to something else and made those updates here. (If these links should be left alone, let me know and I can revert them/feel free to revert them in review.)

Related Issue

• [ ] Closes #
• [ ] Related to #

Checklist

• [X] Checked that the pre-commit linting/style checks pass
• [X] Included tests that prove the fix is effective or that the new feature works
• [X] Added necessary documentation (docstrings and/or example notebooks)
• [X] If you are a pro: each commit corresponds to a relevant logical change

Type of change

• [ ] New feature / enhancement
• [ ] Bug fix
• [X] Documentation
• [ ] Maintenance
• [ ] Other (please specify):