Add documentation about licenses

[ubruhin]

For LibrePCB/LibrePCB#1247, to add a hyperlink to the application pointing to a documentation site explaining the available license options.

Preview: https://docs.librepcb.org/_branches/document-licenses/librepcb/user-manual/licenses/

I'm really not a license expert so I hope these entries are correct so far, and I didn't fill out the CERN-OHL rows yet because I'm not sure (EDIT: filled out with the help of choosealicense.com). Would be nice to get support from someone with more knowledge about these licenses 😉 However, I'll merge anyway soon to get a working hyperlink for the application - content can still be improved later.

Also would be nice to get feedback about some recommendations I've made on this new page:

• Using CC0-1.0 for projects when you want to give other people freedom
• Using CERN-OHL-S-2.0 if you want to avoid closed-source products based on your project (does this make sense?)
• For libraries, only CC0-1.0 should be used (from my point of view)

@dbrgn @rnestler maybe you know more about licensing?

[ubruhin]

Merging now to get the stable URL required for LibrePCB, but still open for feedback 🙂