Add url base exercism.org

[muzimuzhi]

Summary

Some links to https://exercism.org pages are root-relative, which worked only in the website (when markdowns are parsed to html) but not locally (markdown as is).

Popular tracks exercism/java and exercism/python both use non root-relative links to exercism.org, so this change should be safe

• https://github.com/search?q=repo%3Aexercism%2Fpython+docs%2Ftracks%2Fpython&type=code
• https://github.com/search?q=repo%3Aexercism%2Fjava+docs%2Ftracks%2Fjava&type=code

Checklist

• If docs where changed, run ./bin/configlet generate to ensure all documents are properly generated.
• CI is green