Add a 404 page

[HarshKapadia2]

• What is a 404 NOT FOUND HTTP error?
• Create a simple front end 404 template.
  • Create a new CSS file for the page in the ./static/css dir and include it in the {% block head %} section of the HTML file (example) after adding {% block head %}{% endblock %} to ./templates/base.html (example).
• Use Flask error handlers to display a 404 page.

[Haroonchik]

@HarshKapadia2 You can entrust this task to me.

[Haroonchik]

@HarshKapadia2 Check https://github.com/HarshKapadia2/one-or-two/pull/19