docs: fix broken cross-references and section levels in AI docs

- Fix section title levels in configuring-an-ai-provider-api-key.adoc
  (=== to == to follow expected sequence after = heading)
- Add end-user-guide: module prefix to cross-module xrefs in
  ai-provider-api-key-secret-reference.adoc and configuring-ai-providers.adoc
- Remove non-existent aiProviders and defaultAiProvider xrefs from
  checluster-custom-resource-fields-reference.adoc (AI config is
  ConfigMap-based, not CheCluster CR fields)

Assisted-by: Claude Opus 4.6
Signed-off-by: Oleksii Orel <oorel@redhat.com>

Author: olexii4

modules/administration-guide/pages/ai-provider-api-key-secret-reference.adoc

@@ -109,5 +109,5 @@ EOF
 .Additional resources
 
 * xref:configuring-ai-providers.adoc[]
-* xref:mounting-secrets.adoc[Mounting Secrets (end-user guide)]
+* xref:end-user-guide:mounting-secrets.adoc[Mounting Secrets (end-user guide)]
 * link:https://devfile.io/docs/2.2.0/what-is-a-devfile[DevWorkspace Operator documentation]

modules/administration-guide/pages/checluster-custom-resource-fields-reference.adoc

@@ -11,11 +11,9 @@ This section describes all fields available to customize the `CheCluster` Custom
 
 * xref:a-minimal-checluster-custom-resource-example[]
 * xref:checluster-custom-resource-devEnvironments-settings[]
-** xref:checluster-custom-resource-devEnvironments-aiProviders-settings[]
 ** xref:checluster-custom-resource-devEnvironments-allowedSources-settings[]
 ** xref:checluster-custom-resource-devEnvironments-containerBuildConfiguration-settings[]
 ** xref:checluster-custom-resource-devEnvironments-containerRunConfiguration-settings[]
-** xref:checluster-custom-resource-devEnvironments-defaultAiProvider-settings[]
 ** xref:checluster-custom-resource-devEnvironments-defaultNamespace-settings[]
 ** xref:checluster-custom-resource-devEnvironments-defaultPlugins-settings[]
 ** xref:checluster-custom-resource-devEnvironments-editorsDownloadUrls-settings[]

modules/administration-guide/pages/configuring-ai-providers.adoc

@@ -108,4 +108,4 @@ When an administrator updates the registry (for example, removes a tool or chang
 .Additional resources
 
 * xref:ai-provider-api-key-secret-reference.adoc[]
-* xref:using-ai-assistants-in-workspaces.adoc[]
+* xref:end-user-guide:using-ai-assistants-in-workspaces.adoc[]

modules/end-user-guide/pages/configuring-an-ai-provider-api-key.adoc

@@ -18,7 +18,7 @@ Configure an AI provider API key so that it is automatically injected as an envi
 
 You can configure an API key from the *Create Workspace* page or from *User Preferences*.
 
-=== From the Create Workspace page
+== From the Create Workspace page
 
 . Navigate to *Create Workspace*.
 . In the *AI Provider* section, expand *Choose an AI Provider*.
@@ -27,7 +27,7 @@ You can configure an API key from the *Create Workspace* page or from *User Pref
 +
 A *Key configured* badge is displayed on the provider card.
 
-=== From User Preferences
+== From User Preferences
 
 . Navigate to *User Preferences > AI Providers Keys*.
 . Click *Add API Key*.