Move check-doc.py from contrib/devtools to test/lint

Summary:
As per title. This script is intended to be integrated to arc lint in a
later diff, test/lint is a better place for it.

Depends on D2304

Test Plan:

./test/lint/check-dock.py

Check the output is the same than before the move

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2305