Clarify fast-forwarding setup requirements

[simsinght]

Addresses setup questions raised while onboarding a new repository into parallel mode with fast-forwarding.

mergequeue/how-to-guides/fast-forwarding.md

• Sample configuration was not usable as written. preconditions and merge_mode were nested one level too high, and use_fast_forwarding was set to false on the page that tells you to set it to true. Both corrected; the sample now matches the schema in the configuration reference and parses cleanly.
• Explained the protected branch permission. The previous wording said the permission "may" be required without saying how to tell, then said Aviator needs force-push access while not force pushing, which reads alarming without explanation. Now states when the permission is needed, and that the branch update is sent without the force flag so GitHub applies it only as a genuine fast-forward and it cannot rewrite, reorder, or drop commits.
• Documented repository rulesets. The page covered only classic branch protection. Rulesets are evaluated separately, and a "Require a pull request before merging" rule blocks fast-forwarding with an error that does not name the app. Added the error text and the bypass list remedy.

mergequeue/quick-setup.md

Steps 2 and 3 referred to the onboarding flow and the Repositories page with no links. Both now link out.

mergequeue/concepts/parallel-mode/fast-forwarding.md

• The two animations had no captions, so it was not clear which scenario each showed or how they related to the surrounding text. Split the section into passing and failing cases, and captioned each animation.
• Corrected the failure description, which said the first temporary branch is not discarded when its CI fails while simultaneously describing the next branch being rebuilt without those commits.

Not addressed here

The animations themselves are unchanged. They still have no pause control and do not label the branches the text names, which needs new assets rather than a text edit.