docs: document CodecLimits boundary policy

[puffball1567]

Summary

• document that embedding applications choose fixed CodecLimits per trust boundary
• add a concrete bounded-network-payload example
• skip the CI workflow for Markdown/docs-only changes while retaining full CI for source, test, build, configuration, and workflow changes

Verification

• git diff --check

The workflow change intentionally causes this introductory PR to run CI once; future docs-only PRs will not start the matrix.