add ingestion mechanism documentation for rust

[bantonsson]

What does this PR do? What is the motivation?

This adds APM ingestion mechanism documentation for the rust language.

Merge instructions

Merge readiness:

• Ready for merge

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Additional notes

[github-actions]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/bjorn.antonsson/rust-ingestion-mechanisms-sampling/tracing/trace_pipeline/ingestion_mechanisms

[janine-c]

Looks great! Just one formatting inconsistency I noticed, but that's definitely a tiny nit 🙂

[janine-c]

Suggested change

	For example, to collect 100% of the spans from the service named `my-service`, for the operation `http.request`, up to 50 spans per second:
	For example, to collect `100%` of the spans from the service named `my-service`, for the operation `http.request`, up to `50` spans per second:

I noticed we format it this way for the other languages, so we might as well do it here too.