Update Vignette

[zkamvar]

• add mention of model-metadata.json in Configure Modeling Hub.
  • add mention of model-metadata to validate_config()
  • add mention of model-metadata to validate_hub_config()
• swap sections for validate_config() and validate_hub_config().
  • Rationale: people are generally going to be skimming this document, so quick wins presented first (top-down) are going to be more valuable than a bottom-up approach by explaining the components first.
• demonstrate view_config_val_errors() on output from validate_hub_config().
  • Rationale: The sentence "You can also use view_config_val_errors() on the output of validate_hub_config() to review any detected validation errors." occurs at the very end of the vignette and I've already checked out by the time I get there. I did not know that I could do that until Anna told me.

Note: this will also benefit from #32