Documentation: add copy button to code blocks

[deining]

Using the button, you can copy textual content of code block to clipboard.

[codecov-commenter]

Codecov Report

Merging #1198 into master will increase coverage by 0.04%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #1198      +/-   ##
============================================
+ Coverage     94.26%   94.30%   +0.04%     
  Complexity      455      455              
============================================
  Files             2        2              
  Lines          6674     6674              
  Branches       1793     1794       +1     
============================================
+ Hits           6291     6294       +3     
+ Misses          105      102       -3     
  Partials        278      278              

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/picocli/CommandLine.java	94.14% <0.00%> (+0.04%)	314.00% <0.00%> (ø%)

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[remkop]

Merged, and pushed the rendered HTML.

This is very nice! It is subtle but a very nice enhancement for users.
I like this a lot! Thanks for the contribution again!