Readme improvements - Githubissues

Hello!

This PR addresses #39 and #40, and fixes a few things in the README.

Changes done:

Created section for the video tutorials that was missing.
Moved the text refering to the video tutorials from the top of the readme to the appropriate section
Added links to the videos as they were missing.
Clarified step 4 in section 1.3 (Setup your Workspace Repository) to enforce that the < > should be removed (#39)
Added a new entry in the troubleshooting error for the NotImplementedError (#40)

LDSSA / ds-prep-course-2021