docs: fix ECH quickstart architecture note and rebrand miss in apm/otel index

[alexandra5000]

Summary

Fixes two docs contradictions identified in #7874 (items 4 and 5).

Item 4 — ECH quickstart architecture mismatch (quickstart/ech/*.md):
The three ECH quickstarts (hosts/VMs, Docker, Kubernetes) instruct users to use Elastic Agent with the elasticsearch exporter writing directly to Elasticsearch. This contradicts the architecture docs (ECH → Managed OTLP Endpoint) and the modes reference ("the only case where the elasticsearch exporter is recommended" = self-managed gateway).

Rather than rewriting the full quickstarts, added a note admonition to each pointing to the Managed OTLP Endpoint as the recommended path for ECH 9.0+, with a link to the mOTLP quickstart. The existing content remains for users on earlier versions.

Item 5 — Rebrand miss in apm/opentelemetry/index.md:
The 9.5 rebrand updated the {{edot}} substitution value, but this page still contained literal strings: the H2 heading "Why use the Elastic Distributions of OpenTelemetry?" and six SDK link labels ("Elastic Distribution of OpenTelemetry Java/.NET/Node.js/Python/PHP/Browser"). Replaced all with the {{edot}} substitution.

Fixes #7874

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