Ocaml user's manual relicensed under CC BY-SA

Following a recent OCaml developer meeting, the user’s manual has been relicensed under a Creative Commons Attribution-Sharealike license rather than its own specific license.

This new license is already effective on the development version of the manual, and the 4.10 version of the manual would be published with the new license (at the same time as the first 4.10 beta).

This change should make easier to redistribute the manual or explore new designs for the manual.