Update/Improve/Remove doc/compilers
The documentation about compilers seems quite outdated, and also only probably only applies to dune master (does not mention compilers supported by the different dune releases).
Improvements
- If the information is better found somewhere else, remove the page.
- otherwise, list the compiler support in recent dune versions.
- Also update the wording (C++14 is by far not the most recent standard anymore)