[Bf-docboard] License for docs

[Ton Roosendaal]

Hi,

There is a discussion about the proper license for the manual.
https://developer.blender.org/D1948

In short, I suggest:

- Convert all old "Open Content" to CC-BY-SA in the new manual

- Add a license page in the manual that tells that we migrated old OCL content to CC-BY-SA. Tell that the original content is still OCL (link to download), and that the new CC-BY-SA is nearly identical and also copyleft. Add foundation at blender.org email address for people to send questions to if they have.

- For new content advise to use CC-BY-SA, or CC0. At choice of the authors. 

-Ton-

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Ton Roosendaal  -  ton at blender.org   -   www.blender.org
Chairman Blender Foundation, Producer Blender Institute
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