Adds troubleshooting info to get_subprocess_output docs

[lgh2]

What does this PR do? What is the motivation?

Adds troubleshooting information to the get_subprocess_output documentation.

Merge instructions

• [x] Please merge after reviewing

Link to preview: https://docs-staging.datadoghq.com/laura.hampton/update_get_subprocess_output/developers/custom_checks/write_agent_check/#results-1

Additional notes

[github-actions[bot]]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/laura.hampton/update_get_subprocess_output/developers/custom_checks/write_agent_check

[buraizu]

Confirmed that build preview check passed in GitLab.