chore: rename IAM to instance (#11351)

# Which Problems Are Solved

In many cases IAM was used where instance was meant. 

# How the Problems Are Solved

- Rename wrong usage of IAM to instance all over the code base.
- Add an agents.md which explains the terms used in Zitadel's
architecture.

# Additional Context

- closes #11280

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Co-authored-by: Copilot <[email protected]>
Co-authored-by: muhlemmer <[email protected]>
Co-authored-by: Copilot <[email protected]>
Co-authored-by: Marco A. <[email protected]>

apps/docs/content/concepts/architecture/solution.mdx

@@ -31,7 +31,7 @@ Simply steer traffic by path, hostname, IP address or any other metadata to the
 
 ### Single Cluster / Region
 
-A Zitadel Cluster is a highly available IAM system with each component critical for serving traffic laid out at least three times.
+A Zitadel Cluster is a highly available instance system with each component critical for serving traffic laid out at least three times.
 Our storage layer (Postgres) is built for single region deployments.
 Hence our reference design for Kubernetes is to have three application nodes and one storage node.
 

apps/docs/content/concepts/structure/_org_description.mdx

@@ -1,7 +1,7 @@
 ZITADEL is organized around the idea that:
 
 * Multiple organizations can be managed within one [instance](/concepts/structure/instance).
-* organizations can grant each other rights to self-manage certain aspects of the IAM (eg, roles for access management)
+* organizations can grant each other rights to self-manage certain aspects of the instance (eg, roles for access management)
 * organizations are vessels for users and projects
 
 ![](../../../public/img/concepts/objects/organization.png)