Update docs for Private Apps restrictions

[andyw8]

Due to some upcoming platform changes (Aug 25 email), we'll need to update the docs to indicate that the "Manage private apps" option will need to be enabled when setting up Theme Kit on new stores.

The will require updating https://shopify.github.io/themekit/#installation and also the screencast clip.

Some tutorial videos may also need updated @liamgriffin (or we can add an explanatory note?):

• https://www.youtube.com/watch?v=SrtOXwjXJ1w
• https://www.shopify.ca/partners/blog/95401862-3-simple-steps-for-setting-up-a-local-shopify-theme-development-environment

cc @attaboy @gfscott