Edit docs extras syntax highlighting.md

Edit docs extras syntax highlighting.md

Added a bunch of clarifying narrative, looking at the discussion forum for what people are asking about, and what I myself was confused about.

* clearer separation of advantage of each style - server or client side - at the top
* inconsistent newlines (different column widths) so I just removed them from obvious paragraphs
* added that the highlight shortcode is not used for the client-side javascripts
* Hugo is taking crap for Pygments being slow, so tried to emphasize that's it's in Pygment's lap. I got your back, Hugo.
* On client-side added prism example
* More clarity on how the css and js needs to be added to your templates
* Explained how the client-side scripts work