docs(reference): add SeekIndicator API reference page

<!-- drift-type:new-reference --> <!-- trigger-pr:2046 --> <!-- component:SeekIndicator --> ## New Component: `SeekIndicator` A new component `SeekIndicator` was added in #2046 but has no API reference page yet. ### What's needed - [ ] Create reference page with anatomy, prose, demos, and `<ComponentReference component="SeekIndicator" />` - [ ] Add sidebar entry in `site/src/docs.config.ts` - [ ] Verify build: `pnpm -C site build` ### Context - Triggering PR: https://github.com/videojs/v10/pull/2046 - Generated JSON: `site/src/content/generated-component-reference/seek-indicator.json` > Use the `/write-api-reference seek-indicator` skill to scaffold this page.