docs(portal): apply review learnings to B16 custom domain page

- Remove Config Key column from domain variables table (env vars only per learning 21)
- Add SSO doc links in verify step and troubleshooting accordion (learning 3.2)

Author: sharadregoti

portal/install/custom-domain.mdx

@@ -32,10 +32,12 @@ Configuring a custom domain involves three steps:
 
 Two environment variables control custom domain behavior:
 
-| Environment Variable         | Config Key                    | Description                                                                                    |
-| :--------------------------- | :---------------------------- | :--------------------------------------------------------------------------------------------- |
-| `PORTAL_DOMAIN_ENABLE_CUSTOM` | `Domain.EnableCustomDomains` | Enables custom domain support. Set to `true`.                                                  |
-| `PORTAL_DOMAIN_DEFAULT`       | `Domain.DefaultDomain`       | The domain consumers use to access the Live Portal, for example `developers.mycompany.com`.    |
+| 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`. |
+
+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).
 
@@ -145,7 +147,7 @@ After restarting the Portal with the updated configuration:
 
    {/* TODO: Screenshot — Live Portal home page loading on the custom domain with a valid padlock in the browser address bar */}
 
-3. **Test SSO (if configured).** Log in through your identity provider and confirm the callback URL resolves correctly on your custom domain.
+3. **Test SSO (if configured).** Log in through your identity provider and confirm the callback URL resolves correctly on your custom domain. See [Single Sign On](/tyk-stack/tyk-developer-portal/enterprise-developer-portal/managing-access/enable-sso) for SSO configuration details.
 
 ## Update the Custom Domain on an Existing Installation
 
@@ -166,7 +168,7 @@ To change the custom domain after the initial setup:
     Confirm that `PORTAL_DOMAIN_ENABLE_CUSTOM` is set to `true` and that `PORTAL_DOMAIN_DEFAULT` is set to your custom domain. Restart the Portal after making any changes to environment variables.
   </Accordion>
   <Accordion title="SSO callbacks or email links use the wrong host or scheme">
-    The Portal constructs base URLs from the `X-Forwarded-Host` and `X-Forwarded-Proto` headers. If these headers are missing or set incorrectly, callbacks and links will use the wrong host or scheme. Check that your reverse proxy sets both headers on every request.
+    The Portal constructs base URLs from the `X-Forwarded-Host` and `X-Forwarded-Proto` headers. If these headers are missing or set incorrectly, callbacks and links will use the wrong host or scheme. Check that your reverse proxy sets both headers on every request. See [Single Sign On](/tyk-stack/tyk-developer-portal/enterprise-developer-portal/managing-access/enable-sso) for SSO configuration details.
   </Accordion>
   <Accordion title="All users are locked out after login attempts">
     This occurs when `X-Forwarded-For` is not forwarded by the reverse proxy. Without this header, the Portal treats all requests as coming from the proxy IP address and applies rate limits globally. Add `proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;` to your proxy configuration and restart nginx.