Move Code_of_Conduct.rst to the root level. (GH-8209)

Follow up from https://github.com/python/peps/pull/712GH-issuecomment-403591108:

"people using the repo without using GitHub still ought to be reminded of the COC" - GvR
(cherry picked from commit 3ed9192)

Co-authored-by: Mariatta <[email protected]>

Author: miss-islington