Fallback-aware failure handling - example added

[scypio]

Example of fallback aware failure handling ( https://github.com/Cognifide/knotx/issues/466 )

Description

Added new page (fallback.html) that demontrates blank and custom fallback rendering

Motivation and Context

I'd like to showcase new feature in example project

Screenshots (if appropriate)

Upgrade notes (if appropriate)

requires https://github.com/Cognifide/knotx/issues/466

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [X] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• [X] My code follows the code style of this project.
• [X] My change requires a change to the documentation.
• [X] I have updated the documentation accordingly.

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