Replace Fairness diagram with poller-flow flowchart

Mirrors the Priority diagram's poller-polling structure and surfaces
the backlog-only behavior up front: poller checks for backlog, and
only then does the weighted virtual-queue selection apply.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: jpvotta

docs/develop/task-queue-priority-fairness.mdx

@@ -250,7 +250,7 @@ To use Fairness, you need to set fairness keys and optionally fairness weights a
 
 <EnlargeImage
   src="/img/develop/task-queue-priority-fairness/fairness-details.png"
-  alt="High-level of how fairness works"
+  alt="Flowchart of how Fairness selects a virtual queue weighted by fairness weight when the backlog has Tasks"
 />
 
 You can set a Workflow's fairness key and weight via the CLI like so: