Add troubleshooting info on parse error when installing GitLab agent

Merged Brie Carranza requested to merge docs-bcarranza-fix-parse-error-installing-agent into master

What does this MR do and why?

GitLab Support has fielded a few tickets about a specific error message that folks encounter when installing GitLab agent for Kubernetes. This MR updates the Troubleshooting the GitLab agent for Kubernetes docs for the agent to mention this error and provide guidance on resolving it. The goals of this MR are to

  • help deflect tickets like this in the future
  • make tickets about this topic easier to solve

This came up in these tickets that are available to GitLab team members with access to Zendesk:

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Edited by Brie Carranza