Add partner ref. app to sample projects

[chgeo]

No description provided.

[chgeo]

@mrl-sap @ch-sap hope this is fine for you to appear on https://cap.cloud.sap/docs/get-started/samples
Copied the text from your repo's readme. You might want to tweak it.

[mrl-sap]

We are currently working on rephrasing the text, please do not merge yet.

[chgeo]

New text as suggested by @mrl-sap

The Partner Reference Application repository provides you with a “golden path” to becoming a SaaS provider of multi-tenant applications based on the SAP Business Technology Platform (SAP BTP).
The guidance covers building, running, and integrating scalable full-stack cloud applications. It includes an ERP-agnostic design that lets you deliver your application as a side-by-side extension to consumers using any SAP solution, such as SAP S/4HANA Cloud, SAP Business One, and SAP Business ByDesign.

By using BTP services and the SAP Cloud Application Programming Model (CAP), your application meets SAP standards for enterprise-class business solutions. It offers a harmonized user experience and seamless integration, including:

• centralized identity and access management,
• a common launchpad,
• cross-application front-end navigation,
• and secure back-channel integration.

The repository includes the “Poetry Slam Manager” application as a ready-to-run example. It also provides tutorials on how to build the application from scratch using an incremental development approach.
Based on this sample application, you will find the bill of materials and a sizing example. This addresses the question "Which BTP resources do I need to subscribe to and in what quantities?" and serves as a basis for cost calculation.