Let's use CC by-sa 4.0 international as our new default license for
documentation contributed to the developer website. People contributing new
documentation agrees to use this license.

Of course this doesn't necessarily apply to older data which come from various
other sources (the old developer and GUI wikis, the old developer website, the
main website…), unless otherwise stated.

For any data which I created myself in the past (either directly on the
developer website or in the wikis), this commit message is my official statement
that I make it CC by-sa 4.0 international.