Update various Gitaly documents to reflect current state and Praefect compatibility

Update various parts of the documentation with the current state of Gitaly configuration, as well as including the ability to use external Praefect clusters.

• Docs: update charts/globals Gitaly section
• Docs: update multiple external gitaly example
• Docs: update external gitaly documentation to current state
  • Update the external gitaly documentation to the current state
  • Add references to charts/globals#gitaly for appropriate values
  • Switch to using YAML examples instead of --set arguments
  • Add note about possible need to configure global.shell.authToken

cc @zj-gitlab @axil @ljlane

Closes #1895 (closed)