doc: git mv to .md

* doc: rename .markdown references in content
* doc: rename to .md in tools
* doc: rename to .md in CONTRIBUTING.md

PR-URL: https://github.com/nodejs/node/pull/4747


Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: techjeffharris
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>