The specification detailing the great documentation migration
requires a doc/source/contributor/ directory for all contributor
documentation. Keystone had a directory specifically for this type
of content but it was in doc/source/devref/.
This commit renames devref/ to contributor/ because the term
`contributor` is more inclusive of other community members that
aren't necessarily developers. The specification also requires this
convention for consistency with other projects.
Change-Id: Id6363213dc6ba6946bd272ca5ee67b4604afc5ac
Partial-Bug: 1694460
Lance Bragstad