Update handbook: What content goes in documentation and on the blog

Outline that the blog is:

• A marketing channel like Twitter/Facebook/YouTube
• For longer form news on what we have done or are doing
• The type of content is ephemeral, think of it as a longer tweet
• Tutorials should not be put on the blog but the docs and shared in the form of a "longer tweet"

Outline that the documentation is:

• A reference on everything about our product
• For feature coverage, API, University (written and video tutorials)
• The type of content persistent

To do: create issues to review and move tutorials from the blog to documentation. Some tutorials are outdated and will be removed entirely.