lint: promote 'Normative basis' subheading from h3 to h2 (MD001)

Markdownlint's MD001 (heading-increment) disallows h1 → h3 jumps.
Our rule docs open with `# <title>` as the only h1, then proceed
directly to `## Examples` / etc. — my earlier edit inserted a
`### On normative basis` section between them, jumping a level.
Promoted to `##` so the heading tree is h1 → h2 → h2 → ...

Author: johanrd

docs/rules/template-mouse-events-have-key-events.md

@@ -6,7 +6,7 @@ Enforce that `{{on "mouseover" …}}` is accompanied by `{{on "focus" …}}` / `
 
 Keyboard-only users can't trigger mouse events. Pairing hover-in events with focus events (and hover-out events with blur events) ensures the same UI state transitions happen for keyboard navigation.
 
-### On "normative basis"
+## On "normative basis"
 
 [WCAG 2.1 SC 2.1.1 Keyboard (Level A)](https://www.w3.org/WAI/WCAG21/Understanding/keyboard) requires all functionality to be operable via the keyboard. Pointer-only UI transitions (hover effects that show/hide content, highlight rows, etc.) don't satisfy this when no keyboard equivalent exists. However, this rule's specific "hover event + focus event pairing" heuristic isn't literally mandated by the SC — [Understanding 2.1.1](https://www.w3.org/WAI/WCAG21/Understanding/keyboard) allows any keyboard path. The event-pairing convention comes from [WAI-ARIA APG keyboard-interaction guidance](https://www.w3.org/WAI/ARIA/apg/practices/keyboard-interface/) (authoring guidance, not normative), and from all four peer a11y plugins adopting it as the strongest static-analysis proxy. This rule follows the convention.