Quickstart index and EDOT quickstart cleanup

Cleans up the quickstart index and the EDOT quickstart suite to reduce duplication, surface the new deployment × environment matrix as the primary path, and remove superseded pages.

Related to https://github.com/elastic/docs-content-internal/issues/1582

## Changes

- **`quickstarts.md`** — Rewrites the quickstart index. Promotes the {{edot}} quickstarts (deployment × environment matrix: ECH, Serverless, self-managed × Kubernetes, Docker, Hosts) as the recommended path. Moves Agent-based and other quickstarts to an "Other quickstarts" section.

- **`_snippets/explore-your-data.md`** (new) — Extracts the "Explore your data" section into a shared snippet so all 9 new quickstart files use the same content instead of a placeholder.

- **9 new `quickstart/` files** (`ech/`, `serverless/`, `self-managed/` × k8s/docker/hosts_vms) — Replaces the sparse placeholder explore step with the shared snippet. Fixes `description` frontmatter (removes Jinja2 variables).

- **Deleted with redirects** — Two old quickstarts whose content is fully superseded by the new matrix:
- `quickstart-monitor-hosts-with-opentelemetry.md` → redirects to `quickstart/index.md`
- `quickstart-unified-kubernetes-observability-with-elastic-distributions-of-opentelemetry-edot.md` → redirects to `quickstart/index.md`
- Links updated in 7 files that referenced these pages.

- **`quickstart-elastic-cloud-otel-endpoint.md`** — Repositioned from a deprecated quickstart to a focused guide for sending data from any OTLP-compatible collector or SDK to Elastic Cloud using the Managed OTLP endpoint. Removed "Quickstart:" from the title and clarified the audience.

## Out of scope

- Content updates to the existing Agent-based quickstarts.
- `create-metric-threshold-rule.md` / `create-an-inventory-rule.md` Metrics Explorer mentions.