Standardize where to specify design changes in the bug report template (#15027)

Since a lot of bugs end up needing additional product design work, I
propose adding a (commented-out by default) section to this template to
standardize where to add design changes, once settled.

Reasoning: in estimation sessions, it can sometimes be hard to find this
information: sometimes it's in the comments, sometimes it's been added
to the description... either way, its not always obvious to spot. I
think it will help us move quicker if there's a consistent heading to
look for.

(Also, open to suggestions for other ways of wording that heading! This
is just the way I've been adding it to issue descriptions lately.)

.github/ISSUE_TEMPLATE/bug-report.md

@@ -28,3 +28,6 @@ TODO
 N/A
 
 <!-- If this is a performance issue, follow these steps to generate and attach a debug archive: https://fleetdm.com/docs/using-fleet/monitoring-fleet#debugging-performance-issues -->
+
+<!-- ### 🛠️ To fix ### -->
+<!-- If this bug requires additional product design work, uncomment the heading above and add instructions to fix, Figma link, etc. here once design changes are settled. -->