Add known issues for 'no agent found', Agent uninstall exception

[kilfoyle]

This adds docs for the known issues:

• "no agent found", described in Knowledge Center here.
• Fleet-managed agent throws an exception on uninstall in Windows, described in Knowledge Center here.

Closes: #1481 Closes: #1482

I'll update the "Impact" section of the second issue as soon as we have a suggested workaround (Blake or Craig, if you can please let me know once we have that).

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