docs(greenhouse): switch IAM guide to Harvest V3 OAuth credentials

[claude]

Requested by Jeran Norman · Slack thread

Greenhouse is retiring the Harvest v1 and v2 API keys on August 31, 2026, so the IAM guide now walks customers through Harvest V3 OAuth credentials instead of a single API key.

What changed

• Create Harvest V3 (OAuth) credentials in Dev Center and copy the Client ID and Client Secret, which Greenhouse shows only once.
• Enable harvest:users:list and harvest:user_roles:list under Manage Scopes, covering the Users and Roles resources listed in Available data.
• Optionally supply a User ID so calls are made on behalf of a specific Greenhouse user. Left blank, the connection uses the service account attached to the credentials.
• API Reference card now points at harvestdocs.greenhouse.io/docs/authentication; the old developers.greenhouse.io/harvest.html link documents the retiring API.
• Screenshots converted from bare markdown images to the <img> form used by the rest of the guides.
• Admin prerequisite in the top warning now names the "Can manage ALL organization's API Credentials" permission.

Exact-copy (single commit) of #398.

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