Update onboarding

[astrojuanlu]

Addresses #8410 by

1. Directing users to read the Read the Docs tutorial https://docs.readthedocs.io/page/tutorial/ rather than the Getting Started with Sphinx guide
2. Replaces the demo (which was a dead end) by the same template used in the tutorial

This is how it looks like now:

⚠️ Notice that https://github.com/readthedocs/tutorial-template/ doesn't exist yet, until we agree to transfer https://github.com/astrojuanlu/tutorial-template ⚠️

This also removes the demo code.

[humitos]

This is much better UX! 💯

You may need to do a grep in other repositories (in particular -corporate) because there may be views that still depend on this class. Also, you will need to double check that the onboard_import.html file on -corporate does not require a change.

That said, I think we can't merge this PR without checking those things first.

[humitos]

Notice that readthedocs/tutorial-template doesn't exist yet, until we agree to transfer astrojuanlu/tutorial-template

+1 on transfering this

[astrojuanlu]

Good point, will check the corporate site now.

By the way, I don't understand the CircleCI failure 🤔 https://app.circleci.com/pipelines/github/readthedocs/readthedocs.org/2681/workflows/739e8fbc-0291-4bae-b3e9-8a308e633c54/jobs/5268?invite=true#step-105-354 (It's a Codecov problem, the real failure was in the tests)

[ericholscher]

Yay deleting code :)

[astrojuanlu]

Migrated the tutorial template to our GitHub organization https://github.com/readthedocs/tutorial-template/ and replaced another two links on the homepage to point to the Tutorial. Merging!