docs: add env variable AUTH_OIDC_TOKEN_ENDPOINT_AUTH_METHOD (#554)

Co-authored-by: Meier Lukas <meierschlumpf@gmail.com>

Author: d-lindahl

docs/advanced/single-sign-on/index.mdx

@@ -200,6 +200,7 @@ Homarr supports multiple authentication options, from internal userbase (credent
     | ``AUTH_OIDC_NAME_ATTRIBUTE_OVERWRITE`` | Overwrite name attribute. By default it will use preferred_username if it does not contain a `@` and otherwise name. | ---          |
     | ``AUTH_OIDC_FORCE_USERINFO`` | Force userinfo endpoint to be used for user information.    | false         |
     | ``AUTH_OIDC_ENABLE_DANGEROUS_ACCOUNT_LINKING`` | Enable account linking for OIDC provider. This will link the OIDC accounts by email. Be sure that you have verified emails to prevent stealing accounts. | false |
+    | ``AUTH_OIDC_TOKEN_ENDPOINT_AUTH_METHOD`` | Override method for the token endpoint authentication. Supported values are `client_secret_basic`, `client_secret_post`, `client_secret_jwt` and `none`. | client_secret_basic |
 
     ### Permission System