Add page on init container resource calc

[domalessi]

What does this PR do? What is the motivation?

https://datadoghq.atlassian.net/browse/DOCS-10936

Create a page that describes how init container resources are calculated. This is information that only a small subset of users want/need to know, so we want a page to point them to, but we don't need the content to be super visible. I've added a link to it from the Further Reading section of the K8s SSI home page. We can reconsider placement if/when we get feedback to warrant that.

Merge instructions

Merge readiness:

• Ready for merge

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Additional notes

[github-actions]

✅ Documentation Team Review

The documentation team has approved this pull request. Thank you for your contribution!

[github-actions]

Preview links (active after the build_preview check completes)

New or renamed files

• https://docs-staging.datadoghq.com/domalessi/init-resource-calc/tracing/guide/init_resource_calc

Modified Files

• https://docs-staging.datadoghq.com/domalessi/init-resource-calc/tracing/trace_collection/automatic_instrumentation/single-step-apm/kubernetes

[rtrieu]

Some minor feedback for flow and clarity.