a11y(website): make scrollable code blocks keyboard accessible (WCAG 2.1.1)

[ajitpratap0]

Problem

All <pre> / code blocks with overflow-x: auto are unreachable by keyboard — users cannot Tab to them to scroll horizontally. This violates WCAG 2.1.1 (Keyboard, Level A).

axe-core reported 1 affected node on homepage and 13 on the getting-started docs page.

Fix

Add tabIndex={0} to the outer element of the CodeBlock component in website/src/components/docs/mdx-components.tsx. Also fix the hero code block on the homepage.