clarifying licenses

[choldgraf]

This does the following:

• clarifies the licenses in the Data 8 content ecosystem
• specifies that the Data 8 textbook is CC-BY-ND-NC
• removes language that suggests the textbook is open-source, since it is not viewed as an open-source license by most in the open scholarship space
• removes the section about others getting their own copy of the textbook, since this violates the license
• adds information about the other kinds of content and how to access it

Of note - we may also wish to include stronger language that prohibits people from forking and modifying the textbook at all unless they have approval. If anybody forks the textbook on GitHub and makes any change (even if it's just a typo to make a pull-request), they are in violation of the ND clause in the license.

cc @ericvd-ucb and @alexnakagawa in case they have thoughts.

side-note: we should similarly remove language in our other materials that calls the Data 8 textbook an "open source" textbook.

[alexnakagawa]

Thanks so much for clarifying and editing these in. I'll look out for language calling the Data 8 textbook an "open source" one, specifically in the technical infrastructure guide.