Description: Changed the border type such that the horizontal rule only
appears on the bottom and not all around the top bar.

Note: This only applies to the light mode concepts.

Changes:
- Create a README.md file in the concepts directory
- Add the concept images to a table in the README.md file

Changes:
- Create a new "concepts" folder
- Add four renders (course homepage and course view in dark and light
modes) to the concepts directory