docs: redesign landing page with real screenshots and feature showcase (opensearch-project#144)

* docs: redesign landing page with real screenshots and feature showcase

- Add "See It in Action" screenshot showcase section with 6 real product screenshots
  (APM service maps, trace detail, log investigation, agent execution graph,
  Prometheus metrics, cross-signal correlation)
- Expand Features from 4 to 6 blocks: add PPL Query Language and Built-in ML
- Replace mockup visuals with real product screenshots in feature blocks
- Update Hero headline and cycling taglines with feature-focused messaging
- Reorder sections: Hero > Showcase > Features > QuickStart > Integration > WhyOTEL
- Copy product screenshots from starlight-docs to main site public/images/

Signed-off-by: Anirudha Jadhav <anirudha@nyu.edu>

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* test: update CyclingTagline test for new tagline content

Update test assertion from 'Services' to 'Traces' to match the
redesigned cycling tagline labels.

Signed-off-by: Anirudha Jadhav <anirudha@nyu.edu>

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: anirudha