showcase: RestrictInput directive - create a page

[catlabs]

I'm submitting a...

[ ] Regression (a behavior that used to work and stopped working in a new release)
[ ] Bug report  
[ ] Feature request
[x] Documentation issue or request
[ ] Support request => Please do not submit support request here, instead see https://github.com/NationalBankBelgium/stark/blob/master/CONTRIBUTING.md#got-a-question-or-problem

Current behavior

There is no page for this directive in the showcase

Expected behavior

Add a page and an example viewer to demonstrate how this directive works