Remove mostly empty tutorials and codelabs section from sidenav

[parlough]

• Supersedes https://github.com/dart-lang/site-www/pull/4875 to make way for new getting started content.
• Begins consolidating the idea of "codelab" and "tutorial", for a future where we drop codelabs from the site.
• Add codelabs and tutorial not already in sidenav elsewhere to the sidenav.
• Keep tutorial index, but put it in under resources.

Closes https://github.com/dart-lang/site-www/pull/4875

[dart-github-bot]

Visit the preview URL for this PR (updated for commit 5b1c38cb18a7d17562d4dac05affef48fbde136b):

https://dart-dev--pr5858-feat-remove-codelabs-tutorials-top-le-aihyjab5.web.app

[atsansone]

/gcbrun