docs: render component classNames hooks on the docs page (Button + Accordion template)

[Copilot]

Every v9 component exports a <Component>ClassNames object (e.g. buttonClassNames: SlotClassNames<ButtonSlots>) that consumers use for style overrides, but the docsite doesn't surface it — consumers have to read source. This PR wires up a template for two components (Button, Accordion) to agree on placement/styling before fanning out.

Per the issue author's direction, rollout to the remaining ~70 components is intentionally out of scope and will land in a follow-up.

Changes

• react-storybook-addon / FluentDocsPage.tsx — new "Customizing component's slot styles with className hooks" info box, visually uniform with the existing "Native props are supported" and "Customizing components with slots" boxes. Renders a slot → className table from the object passed in on story parameters. Nothing is hardcoded.
• react-storybook-addon / hooks.ts — extends FluentDocsConfig with classNames?: Record<string, string>, read per-story from parameters.reactStorybookAddon.docs.classNames (per-story since each component has its own map).
• Button template — stories/src/Button/index.stories.tsx imports buttonClassNames; table renders root → fui-Button, icon → fui-Button__icon.
• Accordion template — stories/src/Accordion/index.stories.tsx imports accordionClassNames; table renders root → fui-Accordion. Picked as the pair because single-slot validates the layout scales down as well as up.
• Change file — type: none for @fluentui/react-storybook-addon (docs-only). Stories packages are private, no change files needed.

Wiring pattern (duplicated across components in the follow-up)

import { Button, buttonClassNames } from '@fluentui/react-components';

export default {
  title: 'Components/Button/Button',
  component: Button,
  parameters: {
    reactStorybookAddon: {
      docs: { classNames: buttonClassNames },
    },
  },
} as Meta;

Review asks for @microsoft/cxe-prg

• Heading copy ("Customizing component's slot styles with className hooks" + 🙌)
• Table placement (inside the existing info-box stack, above <ArgTypes>)
• Table styling (plain slot / className columns with <code> for the class string, matching the reference screenshot in the issue)
• Config shape (parameters.reactStorybookAddon.docs.classNames) — this is what the ~70 follow-up PRs will use, so worth locking in now

Once shape is agreed, the rollout is a mechanical 3-line diff per component story meta.