feat: Add support for lightboxes

[MoritzWeber0]

Add support for lightboxes to view images in a full-screen mode.

There are three different ways to open a lightbox:

1. Via markdown image. To enable lightbox mode for images, just pass an alt text. Images with alt are considered decorative and therefore don't have a lightbox.

   !["Image caption"]("/path/to/image.webp" "Here is the alt text")
   

2. Via float-image. Same as via markdown image, but the alt text is passed as parameter.

   {{% float-image
       src="image.webp"
       alt="Here is the alt text"
       caption="Image caption"
       width="50%"
       position="right"
   %}}
   

3. Via markdown link. Just pass the reference to the image like you would do with markdown images:

   ["Link text"]("/path/to/image.webp")
   

All of this is done without additional library, just using what we have. A combination of pan & zoom from the country map and the new dialog.

TODO:

• Some parts of the image are cut off
• When dragging out of the screen, the dialog closes
• Use grab / grabbing for pan & zoom

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[github-actions]

Hi there! 👋

Thank you for your contribution to the FIP Guide! 🚀
We appreciate your effort in making rail staff travel information more accessible. 🚄

Checklist before merging:

• Added a description to the Pull Request
• Checked the License of new pictures (non-commercial use without attribution)
• Modified content in English
• Modified content in German
• Modified content in French