update Lab 2 documentation

[cpanato]

Description

update Lab 2 documentation to use arkade to install grafana, and screenshots in how to create the initial dashboard

Fixes https://github.com/openfaas/workshop/issues/206

Motivation and Context

• [ ] I have raised an issue to propose this change (required)

How Has This Been Tested?

I've followed the documentation :)

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [x] My change requires a change to the documentation.
• [x] I have updated the documentation accordingly.
• [x] I've read the CONTRIBUTION guide
• [x] I have signed-off my commits with git commit -s
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.