Add Showcase Application for Sibling Navigation

[robinkuck]

[cla-assistant[bot]]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
2 out of 3 committers have signed the CLA.

:white_check_mark: robinkuck
:white_check_mark: jitendrakansal
:x: Robin Kuck

---

Robin Kuck seems not to be a GitHub user. You need a GitHub account to be able to sign the CLA. If you have already a GitHub account, please add the email address used for this commit to your account.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[jitendrakansal]

@robinkuck Pls. list your app on the showcase apps README. https://github.com/SAP-samples/cloud-mdk-samples/tree/main/Showcase_Apps#applications

[jitendrakansal]

@robinkuck Pls. list your app on the showcase apps README. https://github.com/SAP-samples/cloud-mdk-samples/tree/main/Showcase_Apps#applications

I updated it from my side. Also, added info to display related customers.

[jitendrakansal]

@billfroelich Can you review this PR from your end?