docs(ngrx.io): remove ngrx.io app

[timdeschryver]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/ngrx/platform/blob/main/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
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[x] CI related changes
[x] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

The code for the old ngrx.io website is present and is being build/tested.

What is the new behavior?

This PR removes the old code, and updates the CI pipelines.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

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[rainerhahnekamp]

That's quite a big one :)

[brandonroberts]

[markostanimirovic]

🧹