Move articles/contributors/contrib-* to the shared directory

These files can be used by all languages, except
contrib-develinmemoriam.adoc.

• contrib-386bsd.adoc
• contrib-additional.adoc
• contrib-committers.adoc
• contrib-corealumni.adoc
• contrib-develalumni.adoc
• contrib-portmgralumni.adoc

Reviewed by: bcr, carlavilla
Differential Revision: https://reviews.freebsd.org/D32830