Documenting some native interop things.

[jwill]

Description of what this PR is changing or adding, and why:

This is PR is reimagining some of the Platform Integration section for demonstration purposes. Consider it a draft to advance the conversation.

Issues fixed by this PR (if any):

PRs or commits this PR depends on (if any):

Presubmit checklist

• If you are unwilling, or unable, to sign the CLA, even for a tiny, one-word PR, please file an issue instead of a PR.
• If this PR is not meant to land until a future stable release, mark it as draft with an explanation.
• This PR follows the Google Developer Documentation Style Guidelines—for example, it doesn't use i.e. or e.g., and it avoids I and we (first-person pronouns).
• This PR uses semantic line breaks
  of 80 characters or fewer.