docs: add contributor guide for writing Ironic drivers and interfaces

[cardoe]

What does this change do?

Contributors adding hardware support to ironic-understack currently have
no Ironic-specific onboarding: the hardware-type/interface model, the
provisioning state machine, and the mixin pattern the existing drivers
use are undocumented outside upstream OpenStack docs and the source
itself. Cover that ground, with extra depth on writing an inspect/
management interface for devices that have no BMC or deploy ramdisk
(e.g. network gear), since that's the next class of driver expected.

Upgrade impact

• This change requires operator action to upgrade. If checked, add the
  upgrade-impact label and a release note: run scriv create from the
  repository root and describe the required action in the generated
  changelog.d/ file. See RELEASING.md.

Operator action means anything a deployment has to do beyond a normal resync:
deploy repo or values changes, new or removed secrets, enabling or disabling a
component, or a manual one-time step.