docs: refresh retired training copy

[taigrr]

Summary

• Standardize user-facing references from Github to GitHub
• Fix a README typo and update the old Eventbrite URL to HTTPS
• Use an explicit HTTPS Google Fonts URL in the default layout

Validation

• git diff --check
• Ruby YAML load over _data/users/*

Note: a local jekyll build was not available because this environment lacks Ruby development headers required by the http_parser.rb native gem.