#57 Add documentation for first 7 USD tutorials

[hectorjimenezinbibo]

Issue

Closes #57

Description

This PR addresses the content of the USD tutorials page. This page is intended to how to use Shift for replicating the tutorials from tutorial 1 (Hello World - Creating Your First USD Stage) to Tutorial 7 (Authoring Variants).

This PR contains work related to workflows:

• [x] Create workflow for tutorial 1 & 2
• [x] Create workflow for tutorial 3
• [x] Create workflow/s for tutorial 4
• [x] Create workflow/s for tutorial 5
• [x] Create workflow/s for tutorial 6
• [x] Create workflow/s for tutorial 7

And related to documentation pages:

• [x] Add link in main USD documentation page for the USD tutorials
• [x] Create documentation page for tutorial 1 & 2
• [x] Create documentation page for tutorial 3
• [x] Create documentation page for tutorial 4
• [x] Create documentation page for tutorial 5
• [x] Create documentation page for tutorial 6
• [x] Create documentation page for tutorial 7

[hectorjimenezinbibo]

Please have a look at the tutorials for adding the definite article "the" where you say "Set text as the value of the plug"

I made that change in all the tutorials, I reviewed each step. Please, if some of these cases appear let me know. Thanks!