[Documentation] linux installation suggestions

[Jacinto27]

On the installation's entry for linux it (very helpfully) suggest that if you want formated manpages on hover you can download rmarkdown and pandoc (which comes preinstalled with r-studio). For those who do not have R studio they'd be surprised to learn that pandoc depends on haskell, and thus the base package and it's dependencies can weigh up to 500mbs. I think it's a bit silly to suggest a large package ~just~ for syntax highlighting on function manpage tooltips (something which has native and less heavy alernatives on linux), but I also think the observation is relevant for those who might wonder why their mangpages are in plain text.

I think this can be edited to be part of the installation but not in the order of the default installation steps, but rather a suggestion or a note somewhere else in the doc. I can do this as the pages are editable but I'd like to know if it's ok first!

I believe the changes were made by @18kimn