docs(portal): use config keys as primary in domain config table

Prefer config key names (linked to config reference) over env var names
in the configuration table, with env var listed inline per column.

Author: sharadregoti

portal/install/custom-domain.mdx

@@ -30,16 +30,14 @@ Configuring a custom domain involves three steps:
 
 ## Configure the Portal Domain
 
-Two environment variables control custom domain behavior:
+Two configuration keys control custom domain behavior:
 
-| Environment Variable          | Description                                                                                 |
-| :---------------------------- | :------------------------------------------------------------------------------------------ |
-| `PORTAL_DOMAIN_ENABLE_CUSTOM` | Enables custom domain support. Set to `true`.                                               |
-| `PORTAL_DOMAIN_DEFAULT`       | The domain consumers use to access the Live Portal, for example `developers.mycompany.com`. |
+| Config Key | Description |
+| :--------- | :---------- |
+| [`Domain.EnableCustomDomains`](/product-stack/tyk-enterprise-developer-portal/deploy/configuration) | Enables custom domain support. Set to `true`. Env var: `PORTAL_DOMAIN_ENABLE_CUSTOM`. |
+| [`Domain.DefaultDomain`](/product-stack/tyk-enterprise-developer-portal/deploy/configuration) | The domain consumers use to access the Live Portal, for example `developers.mycompany.com`. Env var: `PORTAL_DOMAIN_DEFAULT`. |
 
-For the equivalent config file keys, see the [configuration reference](/product-stack/tyk-enterprise-developer-portal/deploy/configuration).
-
-Add these variables to your Portal configuration. For how to pass environment variables in your deployment type, see the install guide for [Docker](/portal/install/docker), [Kubernetes](/portal/install/kubernetes), or [Linux](/portal/install/linux).
+Add these to your Portal configuration. For how to pass environment variables in your deployment type, see the install guide for [Docker](/portal/install/docker), [Kubernetes](/portal/install/kubernetes), or [Linux](/portal/install/linux).
 
 ```ini
 PORTAL_DOMAIN_ENABLE_CUSTOM=true