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 Update https://packagemanager.rstudio.com to https://packagemanager.posit.co/ #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 Remove verbose flag by default in vscode.conf #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 pip.conf not being mounted to session containers #423
• Add empty JSON to rstudio-prefs.json default
  • Closes rstudio-prefs.json: {} causing error in every RStudio Pro session log #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 Increasing readinessProbe initialDelaySeconds? #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.