Replace fictional agentops SDK with real opensearch-genai-sdk-py examples and clean up homepage

[ps48]

Summary

• Replace all fictional agentops Python SDK examples on the homepage with real opensearch-genai-sdk-py API (register(), @agent, @tool decorators) and standard OpenTelemetry patterns (OTLPSpanExporter, gen_ai.*
  semantic conventions)
• Update GitHub repo URL from opensearch-project/agentops to opensearch-project/observability-stack
• Remove "Use Cases" from navigation header and consolidate nav items with GitHub link
• Simplify Footer component and update Hero copy
• Update docs get-started page and responsive tests

Changes

• QuickWin.astro — Real SDK quick start with register(), @tool, @agent
• IntegrationPathsTabs.tsx — All 3 tabs updated: GenAI SDK, standard OTEL, and OTLP exporter
• OpenSource.astro — GitHub URL → opensearch-project/observability-stack
• Navigation.astro — Remove "Use Cases" nav item, group nav links with GitHub
• Footer.astro — Simplified layout
• Hero.astro — Copy updates
• Tests — Updated to match new SDK references and URLs

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 18.51%. Comparing base (6aa396d) to head (45020cf).

Additional details and impacted files

@@           Coverage Diff           @@
##             main       #8   +/-   ##
=======================================
  Coverage   18.51%   18.51%           
=======================================
  Files           3        3           
  Lines          54       54           
  Branches       18       19    +1     
=======================================
  Hits           10       10           
  Misses         44       44           

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