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/examples/login/go.md

@@ -122,7 +122,7 @@ https://github.com/zitadel/zitadel-go/blob/next/example/app/templates/profile.ht
 ### Start your application
 
 You will need to provide some values for the program to run:
-- `domain`: Your ZITADEL instance domain, e.g. my-domain.zitadel.cloud
+- `domain`: Your ZITADEL custom domain, e.g. my-domain.zitadel.cloud
 - `key`: Random secret string. Used for symmetric encryption of state parameters, cookies and PCKE.
 - `clientID`: The clientID provided by ZITADEL
 - `redirectURI`: The redirectURI registered at ZITADEL

docs/docs/examples/login/symfony.md

@@ -375,7 +375,7 @@ OIDC_CLIENT_ID="248680248240075805@dev"
 OIDC_CLIENT_SECRET="BJPhEJULSUXseC4geqg5Yg4wWMoy7RgZKar86mbIpt8ZekC5kixMzYGcXLDeeJv7"
 ```
 
-> The well-known URL needs to be adjusted to your own instance domain.
+> The well-known URL needs to be adjusted to your own custom domain.
 
 Activate the route that is used as callback by the OIDC bundle:
 

docs/docs/examples/secure-api/go.md

@@ -98,7 +98,7 @@ https://github.com/zitadel/zitadel-go/blob/next/example/api/http/main.go
 ```
 
 You will need to provide some values for the program to run:
-- `domain`: Your ZITADEL instance domain, e.g. https://my-domain.zitadel.cloud
+- `domain`: Your ZITADEL custom domain, e.g. https://my-domain.zitadel.cloud
 - `key`: The path to the downloaded key.json
 - `port`: The port on which the API will be accessible, default it 8089
 

docs/docs/examples/secure-api/python-flask.mdx

@@ -179,10 +179,11 @@ class ZitadelIntrospectTokenValidator(IntrospectTokenValidator):
         res = self.introspect_token(*args, **kwargs)
         return res
 ```
-3. Create a new file named ".env" in the directory. Copy the configuration in the [".env.example"](https://github.com/zitadel/example-api-python3-flask/blob/main/.env.example) file to the newly created .env file. Set the values with your Instance Domain/Issuer URL, Client ID, and Client Secret from the previous steps. Obtain your Issuer URL by following [these steps](/docs/guides/start/quickstart#referred1).
+
+3. Create a new file named ".env" in the directory. Copy the configuration in the [".env.example"](https://github.com/zitadel/example-api-python3-flask/blob/main/.env.example) file to the newly created .env file. Set the values with your Custom Domain/Issuer URL, Client ID, and Client Secret from the previous steps. Obtain your Issuer URL by following [these steps](/docs/guides/start/quickstart#referred1).
 
 ```python
-ZITADEL_DOMAIN = "https://your-domain-abcdef.zitadel.cloud"
+ZITADEL_DOMAIN = "https://custom-domain-abcdef.zitadel.cloud"
 CLIENT_ID = "197....@projectname"
 CLIENT_SECRET = "NVAp70IqiGmJldbS...."
 ```