Make use of theme vs2015 for hilight.js and minor other improvements

[AlexAndBear]

Fixes: #775 and #797

This PR improves the appearance of code blocks.

• Changed the Copied button to be a cursor
• Changed the text Copied! --> Copied !
• Improved language used and copied text location
• Made the theme for Highlight JS selectable
• Fixed CSS code to make the selected theme correctly visible
• Improved font sizes in source blocks for hljs
• Added a border radius for the code block

[mmattel]

Locally tested:

• yarn install
• yarn bundle
• Built full docs via yarn antora-bundle

✔️ All steps are free of errors ✔️ Random samples in various docs if code blocks are rendered correctly

Only one source block using callouts (general-info in docs-ocis) does not render nice. I will fix that in an independent PR.