Add otel distro links to APM book

[bmorelli25]

No description provided.

[colleenmcginnis]

Some thoughts on integrating mentions of the Elastic distros into this guide:

Compare data ingestion options

Somewhere in the APM docs, should we compare the APM agents vs. Elastic Distros for OpenTelemetry vs. OpenTelemetry integration? Whenever there's more than one way to do something, there are likely to be questions about which should be used in different contexts/use cases/preferences. How will users know which to choose if they haven't come in with an existing opinion (or are we assuming they will)?

Consistently present Elastic distros as an alternative to APM agents

Throughout the observability guide, we tend to say "APM agent" to mean "the thing you use to ingest data". For example:

• Getting started content instructing users to install APM agents
• Diagrams used to explain how APM works

Where we use "APM agent(s)" to mean "the thing you use to ingest data" (and not specifically one of the Elastic APM agents) should we acknowledge that we mean "Elastic APM agents and/or Elastic distros"? Or is "APM agent" meant to be a broad term that encompasses both? If it's not, we'd probably want to establish a single term that encompasses both.