Add copy button to code blocks

[weiglszonja]

Motivation

Add copy button to code blocks in the documentation.
The button is customised to be always visible but faded (see below), it becomes fully visible when mouse is hovered over.

resolves #1458

Checklist

• Did you update CHANGELOG.md with your changes?
• Have you checked our Contributing document?
• Have you ensured the PR clearly describes the problem and the solution?
• Is your contribution compliant with our coding style? This can be checked running flake8 from the source directory.
• Have you checked to ensure that there aren't other open Pull Requests for the same change?
• Have you included the relevant issue number using "Fix #XXX" notation where XXX is the issue number? By including "Fix #XXX" you allow GitHub to close issue #XXX when the PR is merged.

[codecov]

Codecov Report

Merging #1460 (df770ba) into dev (d3f9f15) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev    #1460   +/-   ##
=======================================
  Coverage   77.46%   77.46%           
=======================================
  Files          37       37           
  Lines        2733     2733           
  Branches      455      455           
=======================================
  Hits         2117     2117           
  Misses        535      535           
  Partials       81       81           

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[oruebel]

Looks good to me. That's a really useful feature!

[weiglszonja]

Thanks @oruebel :)