Docs: fix overflow:auto horizontal scrollbars covering last line of code blocks ...

Docs: fix overflow:auto horizontal scrollbars covering last line of code blocks (#37694)

* Add dirty hack for Firefox

* Disable stylelint check for function-url-quotes

to make the hack go through unchallenged

* Make the fix across all browsers

It's not just Firefox - same issue can be seen (though slightly less horrible looking) on macOS / Chrome etc

* Tweak padding, move clipboard button down slightly

* Further CSS tweaks

Forcing `overflow: overlay` should normalise behaviour between Chrome/Win and other implementations. While visually the Chrome/Win scrollbar still looks big and ugly, its height/spacing is now taken into account as being part of the content, so styles can be applied consistently with extra padding at the bottom

* Linting fixes

* Linting fixes

* Apply suggestions from code review

---------

Co-authored-by: Mark Otto <markd.otto@gmail.com>