[Website]: Managed OTLP docs don't mention `elasticsearch.index` routing

### Before you submit

- [x] This issue is about a documentation page, flow, or piece of content.

### Type of issue

Missing information

### What documentation page or section is affected

https://www.elastic.co/docs/reference/opentelemetry/motlp

### What happened?

https://www.elastic.co/docs/reference/opentelemetry/motlp#routing-telemetry-to-dedicated-datasets-or-namespace documents how to route to different data streams by setting the `data_stream.dataset` attribute. It does not mention that you can also route to arbitrary* indices by setting `elasticsearch.index`, as described at https://github.com/open-telemetry/opentelemetry-collector-contrib/tree/main/exporter/elasticsearchexporter#elasticsearch-document-routing

We should document that. We'll need to be clear that routing to an index like this means that you're on your own as far as index templates / mappings go.

*arbitrary so long as they start with "logs", "metrics-", or "traces-"

### Additional info

_No response_

Open the Managed OTLP documentation at the linked page and read the routing telemetry to dedicated datasets or namespace section. Compare it with the linked OpenTelemetry Elasticsearch exporter documentation, then document routing with `elasticsearch.index`, including the allowed index prefixes and the responsibility for index templates and mappings. Done means the page explains both routing options and their tradeoffs.

Written by the indexing model from the issue text.