docs: add sphinx-copybutton for code blocks (#3493)

Added sphinx-copybutton extension to make it easier for users 
to copy code snippets directly from the documentation.

Changes:
- Added sphinx-copybutton to extensions in doc/conf.py
- Added sphinx-copybutton==0.5.2 to requirements/docs.txt

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Co-authored-by: Enrico Minack <github@enrico.minack.dev>