Make images with captions prettier

[onnimonni]

Changes

I saw you used {.text-center} in some example content directly from the markdown but maintaining these becomes pretty unsustainable easily.

I added sensible pretty styles for images with captions like:

![German green emission sticker](https://www.germanemissionssticker.com/img/sites/16/germanemissionssticker.jpg "Example German green emission sticker for EURO 4 emission standard")

I included example screenshots from my site:

After this PR

Light mode mobile

Light mode

Dark mode

Before this PR

Dark mode

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

[netlify[bot]]

👷 Deploy request for hugo-lotusdocs pending review.

Visit the deploys page to approve it

Name	Link
🔨 Latest commit	22f440659c44c2f84886cc98f87425670b79b476