[8.15] Add docs for agent 'otel' mode (backport #1250)

[mergify]

This is a redo of #1095 reducing the docs that we include in the Fleet & Elastic Agent Guide. Full docs for running Elastic Agent as an OTel collector can be found in the elastic/opentelemetry repo.

This updates the F&A Guide with:

• A brief intro: Run Elastic Agent as an OTel Collector
• The elastic-agent otel command

FYI @ycombinator & @strawgate

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This is an automatic backport of pull request #1250 done by Mergify.

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