Add docs for the fairer task queue

[droyad]

What changed

• New page Tasks ➜ Fairer task queue (fairer-task-queue.md) explaining the fair priority task ordering shipped in Octopus 2026.2 (Server PR #41192, internally the fair-priority-task-orderer feature toggle).
• Cross-link added to the Task queue priority section of prioritize-tasks.md, since this feature changes the ordering rules that section describes.

Why

The fairer task queue is about to roll out to all customers, and nothing in the docs explains it. When multiple projects have queued priority tasks, Octopus now divides the task cap equally between them — ceiling(cap ÷ projects) — and runs priority tasks from under-quota projects ahead of projects already at their share, so one project's bulk multi-tenant rollout can't monopolize the queue for hours.

Content is based on the implementation and XML docs in Octopus.Server/Orchestration/TaskQueue/FairPriorityTaskOrderer.cs; the worked example mirrors the RFC scenario pinned by the unit tests, and the self-hosted environment variable name was verified against the toggle provider code.

Notes for reviewers

• Rollout wording: the page says the feature is "being rolled out progressively to all customers" and documents the self-hosted env var (OCTOPUS__FeatureToggles__FairPriorityTaskOrdererFeatureToggle). If the plan is default-on in a specific release, that callout and the Availability section should be reworded. The toggle is slated for removal by end of Q4 2026, at which point the Availability section can go.
• Move to Top nuance: the new page states that Move to Top tasks are subject to the same per-project sharing (they are — weight 2000 still counts as a priority task in the orderer). This slightly weakens the existing "prioritizes the selected task above all others" promise on the Prioritize Tasks page, so worth a nod from Backend Foundations that this is the intended customer-facing message.

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