Add general not found page

[AlexAndBear]

Description

We've added a not found page to communicate to users when they've navigated to a page that doesn't exist.

Related Issue

• Fixes Not found page #5804

Motivation and Context

How Has This Been Tested?

• test environment:
• test case 1:
• test case 2:
• ...

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Technical debt
• Tests
• Documentation
• Maintenance (e.g. dependency updates or tooling)

Open tasks:

• ...

[update-docs]

Thanks for opening this pull request! The maintainers of this repository would appreciate it if you would create a changelog item based on your changes.

[kulmann]

Love it! 😍 Care to add an e2e test for the page?

[sonarcloud]

Quality Gate failed

Failed conditions
1 New Bugs (required ≤ 0)

See analysis details on SonarCloud

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[kulmann]

Could you add a second one for "the public" (i.e. anonymous access)? Important that the page also works unauthenticated.

[kulmann]

As we found out in a meeting: we can't offer the notFound page in an anonymous context at the moment: user doesn't get initialized and more importantly, apps don't get initialized in an anonymous context. We might change that in the future, but for now we need to accept that the notFound page only works in user- and publicLink-contexts.