docs: use StackBlitz WebContainers for live examples

[cedricduffournet]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/ngrx/platform/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Documentation has been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[x] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[x] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Closes #3261

What is the new behavior?

Target web containers in stackblitz live example (more info about stackblitz webcontainer here)

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

• took example on this PR in angular repo
• this PR fix issue in store testing example (currently not working). Test are not executed automatically. Must execute yarn test command in webcontainer
• I could now make some research on how we could make example-app working in stackblitz as webcontainers support TS paths (see discussion here)

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[markostanimirovic]

Great work @cedricduffournet! 🥇

[timdeschryver]

This is great, thanks @cedricduffournet and thanks for the cleanup!

[timdeschryver]

Closing and re-opening to retrigger the CI

[markostanimirovic]

Rebased onto the master branch to trigger circleci: test check.

[markostanimirovic]

Thanks @cedricduffournet!

Btw, the component-store-slide-toggle example also works again! 🎉

[cedricduffournet]

Nice ... I didn't know this example did not work