O11y rework: Landing page

## Definition of done
- https://developer.konghq.com/observability/
- [ ] Cards/tiles/sections/etc. for the three main areas (debug runtime, analyze APIs, third-party observability)
- [ ] Debug section contains debugger info/tiles
- [ ] Analyze APIs contains Advanced Analytics content
- [ ] Third-party contains stuff like o11y plugins (OTEL, Prometheus, etc.) and their how tos
- [ ] Remove “custom dashboard” section, move that content to the custom dashboard reference instead
- [ ] Add use cases table (primarily from old docs with a few tweaks)
- [ ] Before Features section, add use case section to highlight existing custom dashboard and LLM analytics (ex. [Visualize LLM traffic with Prometheus and Grafana](https://developer.konghq.com/how-to/visualize-llm-metrics-with-grafana/)) use case how-tos

**Notes:**
Possible reference: https://docs.statsig.com/product-analytics/overview
“i like their overview page and how they talk about the different capabilities at the high-level but also how they mention charts. the pages that explain the difference features are also pretty well written.”

## Information
DRP: Christian H.

## Size
M

Start with the observability landing page at https://developer.konghq.com/observability/ and review the old documentation for the use cases and custom dashboard content. Use the Statsig overview page as the stated reference, and inspect the linked LLM metrics how-to. Done means the page covers the three observability areas, includes the requested use-case content, and moves custom dashboard material to its reference.

Written by the indexing model from the issue text.