Draft of modeling ragged time-varying hierarchies (#3676)

## What are you changing in this pull request and why?
Resolves #3328 


## Checklist
<!--
Uncomment if you're publishing docs for a prerelease version of dbt
(delete if not applicable):
- [ ] Add versioning components, as described in [Versioning
Docs](https://github.com/dbt-labs/docs.getdbt.com/blob/current/contributing/single-sourcing-content.md#versioning-entire-pages)
- [ ] Add a note to the prerelease version [Migration
Guide](https://github.com/dbt-labs/docs.getdbt.com/tree/current/website/docs/guides/migration/versions)
-->
- [ ] Review the [Content style
guide](https://github.com/dbt-labs/docs.getdbt.com/blob/current/contributing/content-style-guide.md)
and [About
versioning](https://github.com/dbt-labs/docs.getdbt.com/blob/current/contributing/single-sourcing-content.md#adding-a-new-version)
so my content adheres to these guidelines.
- [ ] Add a checklist item for anything that needs to happen before this
PR is merged, such as "needs technical review" or "change base branch."

Adding new pages (delete if not applicable):
- [ ] Add page to `website/sidebars.js`
- [ ] Provide a unique filename for the new page

Removing or renaming existing pages (delete if not applicable):
- [ ] Remove page from `website/sidebars.js`
- [ ] Add an entry `website/static/_redirects`
- [ ] [Ran link
testing](https://github.com/dbt-labs/docs.getdbt.com#running-the-cypress-tests-locally)
to update the links that point to the deleted page

---------

Co-authored-by: Joel Labes <joel.labes@dbtlabs.com>