Move 2019 Hacktoberfest from upcoming to past events section

[akbernamazi]

Contributing guidelines

• Always run and test the site through lighthouse and webhint.
• Use BEM style CSS class names.
• Use concise 2 spaces around the codebase (sorry, no tabs).
• Don't use spaces in Markdown for new line, explicitly write new lines.
• All images need to be processed based on the guide: https://images.guide/#the-tldr.
• Bonus points if you follow commitizen style commit messages.