Update images from "Build a dApp in 15 minutes" to be better integrated in the dark theme

[andrei-x1]

Description of the pull request (what is new / what has changed)

Build a dApp in 15 minutes

• update previously improved diagrams to look better integrated in the existing dark theme (continuation for https://github.com/multiversx/mx-docs/pull/833/files) -> transparent background
• reduced the width of screenshots to have a more readable preview (otherwise the font is too small)

Did you test the changes locally ?

• yes
• no

Which category (categories) does this pull request belong to?

• document new feature
• update documentation that is not relevant anymore
• add examples or more information about a component
• fix grammar issues
• other

[andrei-x1]

Please review:
@bogdan-rosianu
@ssd04
@andreibancioiu