Updated README.md and .md files Repo Wide

[samforstot]

I felt that the documentation across the repo could be a bit more consistent in order to make it seem more engaging!
Changes:

• Added graphics to pages to make the pages seem more consistent
• Updated wording across the .mds (didn't want to step on too many toes though because of the user created lessons)
• Added a course track so you can navigate to lessons from the main page and see the path clearer
• Updated the contribution .md to reorganize the contribution guidelines