docs: add Graph Insights user guide

[MaximilianWenzel]

Introduces the user guide for the Graph Insights extension. Includes:

• Core Documentation: Step-by-step supply chain tutorial, core concepts, and comprehensive feature references (concepts.md, tutorial.md, features/*.md).
• Integration Guide: Added navigating-to-cmem.md to document backlinks (jumping from a resource/class back to CMEM).
• Index Updates: Appended the user guide introduction to the existing index.md.

Note for reviewers:
Please note that these user-guide documentation files (concepts, tutorial, features) are auto-generated from our single-source pipeline since it is used for different variants of our application.
A _README.md is included in the directory detailing which files are safe to edit manually (e.g., index.md and navigating-to-cmem.md) and which will be overwritten during our next automated sync. Therefore, if you need text or terminology updates in the generated files, please notify us so that we can update our templates and retrigger the build pipeline.

Shared integration documentation:
We have documented the outbound navigation (Graph Insights -> CMEM). Since the inbound buttons (CMEM -> Graph Insights) reside in your native UI, we recommend your team to add a brief section in your core platform documentation covering how users initiate a graph exploration.

Let me know if you need any adjustments.