WIP Extend RStudio documentation

[helijuottonen]

Proposed changes

Preview: https://csc-guide-preview.2.rahtiapp.fi/origin/rstudioupdate/computing/webinterface/rstudio/

Extended web interface RStudio documentation:

• added a bit more explanation on OMP_NUM_THREADS
• information on what RStudio is meant for and that heavier work should be batch jobs
• added FAQ for 3 most common RStudio questions: what to do when RStudio doesn't start, how to install packages, how to change the directory shown in the file panel

Checklist before requesting a review

• I have followed the instructions in the Contributing and Styleguide documents.
• My pull request passes the automated tests. If not, visit pull requests at Travis CI and have a look at the job log.
• [x ] I have included a link to the appropriate preview page (select your branch from the list).

[jessepharrison]

Very useful changes overall! One might consider adding a screenshot of the files pane (showing where exactly one can change directories).