Add tutorial and challenge for indexing lists

[sabderemane]

1. This pull request resolves #13

2. Description

• Create tutorial with comment for use indexes with lists
• Create a challenge with assertions

3. Fill in checklist by marking [x]

• [x] I've read the CONTRIBUTING.md
• [x] I was assigned to this issue
• [x] My code is formatted using required linting
• [x] I've run my code locally and checked it works

[funbeedev]

@sabderemane very nice tutorial and challenge :)