add semantic models

[emmyoop]

resolves #431

Description

Checklist

• I have read the contributing guide and understand what's expected of me
• I have signed the CLA
• I have run this code in development and it appears to resolve the stated issue
• I have opened an issue to add/update docs, or docs changes are not required/relevant for this PR
• I have run changie new to create a changelog entry

[github-actions]

Thank you for your pull request! We could not find a changelog entry for this change. For details on how to document a change, see the contributing guide.

[emmyoop]

Metrics Page - depends on a semantic model

Semantic Model Page with new Project nav on left

condensed DAG model -> semantic model -> metric

full screen view of above

[emmyoop]

A few observations:

1. semantic models don't have labels so there is no "friendly name" to display. Used node.name instead.

Open Questions

1. Is there some kind of code to display? Metrics used to display the generated sql but dbt docs is failing on that. Wasn't sure if it's not possible or if I missed something.
2. Would you prefer a different color for semantic models?
3. Should other fields be displayed for semantic models? The above is a copy of the metric node page.

[emmyoop]

Added entities

[aranke]

Not the most familiar with this codebase, but at a glance LGTM!