Improve several default settings and add to README for Workbench

[tnederlof]

This PR fixes/improves several rough edges in the Workbench chart. @SamEdwardes and I went through them together and each tested the changes.

• Update default package manager repo in config.session.repos.conf
  • Closes https://github.com/rstudio/helm/issues/418 and changes to CRAN so the default gets picked up in RStudio Pro sessions
• Remove --verbose from args in config.server.vscode.conf
  • Closes https://github.com/rstudio/helm/issues/461 as this was removed in Workbench default config at some point but not in the helm values
• Add instructions for configuring R and Python repos in the README
  • Documents how to set repos and mount pip.conf to close https://github.com/rstudio/helm/issues/423
• Add empty JSON to rstudio-prefs.json default
  • Closes https://github.com/rstudio/helm/issues/468
• Add documentation for databricks.conf
  • Documents in the README that databricks.conf should be put in config.secret
• Increase readinessProbe.initialDelaySeconds to 10
  • Closes https://github.com/rstudio/helm/issues/253 as the startup script often takes 7-8 seconds

[SamEdwardes]

I did a test deployment with these changes:

# test-values.yaml

homeStorage:
  create: true
  mount: true
  storageClassName: efs-client
  requests:
    storage: 8G

sharedStorage:
  create: true
  mount: true
  storageClassName: efs-client
  requests:
    storage: 1G

userCreate: true

helm upgrade --install samedwardes-workbench-test . --set-file license.file.contents="workbench.lic" --values test-values.yaml

I was able to start each IDE without issue. LGTM.