New docs: document use of APIs for copying files to/from containers

[rnorth]

Improvement for the new mkdocs-based documentation.

• Add to docs, with examples brought in via codeinclude.
• Explain preference for using the copy APIs instead of volume mounts

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. If you believe this is a mistake, please reply to this comment to keep it open. If there isn't one already, a PR to fix or at least reproduce the problem in a test case will always help us get back on track to tackle this.