Add developer docs for keystone-manage doctor
Add developer docs for keystone-manage doctor

``keyestone-manage doctor`` is a thing, but we don't really provide
any sort of developer documentation on how to write checks for it. We
should have some developer docs that describe the basic steps for
writing new symptoms ``doctor`` should check for.

Change-Id: I98171d5a81cab10020cbaa166bc649bacf7d5a67
Closes-Bug: 1641623

openstack/keystone,rajalokan/keystone,openstack/keystone,openstack/keystone,rajalokan/keystone,rajalokan/keystone,ilay09/keystone,mahak/keystone,mahak/keystone,ilay09/keystone,mahak/keystone,ilay09/keystone