docs: Getting Started Improvements

[CharlieTLe]

• Enable auth for multi-tenancy.
• Load the Cortex dashboards from the cortex-jsonnet repo for the getting-started tutorial. This should allow newcomers to get a better feel for what Cortex is doing.
• Configure recording rules, alerting rules, and alertmanager with cortextool.
• Add Grafana datasources to showcase multi-tenancy.

What this PR does:

Which issue(s) this PR fixes: Fixes #

Checklist

• [ ] Tests updated
• [ ] Documentation added
• [ ] CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]