Allow to add noindex meta tag via frontmatter

[codablock]

Changes

This allows to set noindex: true via frontmatter.

Dark mode

• The UI has been tested both in dark and light mode
• This PR does not change the UI

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[codablock]

fyi, this was my naive approach to add noindex to one of my pages. I'm not that experienced with Hugo, so I might have missed the "correct" way to do it and I'm happy if someone gives me a hint on how to do it properly :)