docs(inconsistent naming): custom domain (#10377)

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# Which Problems Are Solved

partially #9342

# How the Problems Are Solved

Name custom domain consistently in the docs according to
https://github.com/zitadel/zitadel/issues/5888

# Additional Changes



# Additional Context

docs/docs/self-hosting/manage/cache.md

@@ -161,7 +161,7 @@ Caches:
 All HTTP and gRPC requests sent to ZITADEL receive an instance context. The instance is usually resolved by the domain from the request. In some cases, like the [system service](/docs/apis/resources/system/system-service), the instance can be resolved by its ID. An instance object contains many of the [default settings](/docs/guides/manage/console/default-settings):
 
 - Instance [features](/docs/guides/manage/console/default-settings#features)
-- Instance domains: generated and [custom](/docs/guides/manage/cloud/instances#add-custom-domain)
+- Custom domains: generated and [custom](/docs/guides/manage/cloud/instances#add-custom-domain)
 - [Trusted domains](/docs/apis/resources/admin/admin-service-add-instance-trusted-domain)
 - Security settings ([IFrame policy](/docs/guides/solution-scenarios/configurations#embedding-zitadel-in-an-iframe))
 - Limits[^2]