fix(docs): prevent hero-text select label flicker on reload (#430)

Radix Select injects the selected option label into the trigger via a
client-only portal, so the statically rendered page ships an empty
trigger and the label pops in after hydration. Render the value as
SelectValue children instead, so the label is present in the SSR HTML
(this also disables the portal path via valueNodeHasChildren).

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>