Add documentation about setting r-versions in Workbench

[tnederlof]

Some customers need to set the r-versions config file to customize certain R version sessions as documented here.

Nothing in the README mentions it but it is possible to customize as it is a file that just needs to make its way into the /mnt/session-configmap/rstudio directory. It is a tricky format that is not like the rest of Workbench config files so it needs to be in a form like the following in a values.yaml file (with care taken to preserver the Key: Value format and new lines between each section or it will be ignored):

config:  
  session:
    r-versions: |
        Path: /opt/R/4.1.3
        Label: Custom 4.1.3
        Repo: https://packagemanager.posit.co/cran/__linux__/jammy/latest

        Path: /opt/R/4.2.3
        Label: Custom 4.2.3
        Repo: https://p3m.dev/cran/__linux__/jammy/latest

[tnederlof]

@npelikan fyi!