Reorganise r-env documentation

[helijuottonen]

Proposed changes

Big re-organisation of r-env documentation

1. Shortening the main r-env page and removing some less relevant content. Aim is to help people find the most important information more easily, and the page had grown too long.
   https://csc-guide-preview.2.rahtiapp.fi/origin/renv_big_update/apps/r-env/

2. Moving all examples of parallel batch jobs to separate page and adding separate tabs for Puhti and Mahti (and Roihu to be added soon). Idea here is that people who know what they are doing are able to add more cores based on the basic batch job script template in the main documentation, and others need more information. Some examples have been updated, especially MPI examples to emphasize that they are meant for jobs using multiple nodes and more cores than a single node has. For example, to take into account that it is unnecessarily complicated to use MPI with R for something that needs 8 cores.
   https://csc-guide-preview.2.rahtiapp.fi/origin/renv_big_update/support/tutorials/parallel-r-examples/

3. Small updates to the parallel R tutorial:
   https://csc-guide-preview.2.rahtiapp.fi/origin/renv_big_update/support/tutorials/parallel-r/

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.
• I have included a link to the appropriate preview page (select your branch from the list).

[helijuottonen]

Closed because I managed to create a Git mess. Will create a new pull request for this.