feat: nginx sec headers

[Lennart01]

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• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• The light- and dark-theme are both supported and tested
• The design was implemented and is responsive for all devices and screen sizes
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[github-actions]

The deployment to the dev cluster was successful. You can find the deployment here: https://4397.development.scrumlr.fra.ics.inovex.io
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[Schwehn42]

I'm usually a proponent of splitting stuff into multiple files, however I'm not sure in this case because it adds another file to the project root. Could you add in into the main file or if you want to split it maybe add a subdirectory?