Update to glimmer components

[miguelcobain]

Let's use this PR to buffer the changes to update the components (or should we merge them as they come?).

Currently migrated DocsDemo (and sub-components), DocsSnippet and DocsCodeHighlight.
Also started a dedicated docs page to the v5 breaking changes/upgrade instructions.

[SergeAstapov]

Thank you @miguelcobain!

[RobbieTheWagner]

This looks awesome, thanks @miguelcobain! Should we go ahead and merge this?

[miguelcobain]

@rwwagner90 I think merging this to master might be a bit imprudent.
This PR contains breaking changes, so merging it to master means that we wouldn't be able to easily make patch releases while this update is made.

I think merging this to a v5 branch or something similar could be a good middle ground.

[RobbieTheWagner]

@miguelcobain I'm not worried about introducing breaking changes to master. I don't plan on any patch releases before releasing a new major version. I just wanted to double check with you that this was ready to go, since it sounded like you weren't sure if you wanted to do one PR for all glimmer conversions or a few.

[miguelcobain]

@rwwagner90 this PR should be completely functional. The migrated components do work and the all docs and tests were updated according to that migration. No holes here.

I think separate PRs makes it easier for you guys to review. But I can go either way. Whatever you prefer.

[RobbieTheWagner]

@miguelcobain I went ahead and merged, thanks for your work!