[docsy] Cleanup Hugo config, add preliminary inline-style workaround

[chalin]

• Followup to Docsy-based version of website #53
• Contributes to [docsy] Work around inline style and script CSP restrictions #55
• Cleans up hugo.yml, removing unnecessary config inherited from the Docsy example
• Adds preliminary Content-Security-Policy for navbar logo. More will follow, but we need to update Hugo first.

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