feat: add custom docs URL to deployment config (#8590)

cli/testdata/coder_server_--help.golden

@@ -128,6 +128,9 @@ Use a YAML configuration file when your server launch become unwieldy.
       --access-url url, $CODER_ACCESS_URL
           The URL that users will use to access the Coder deployment.
 
+      --docs-url url, $CODER_DOCS_URL
+          Specifies the custom docs URL.
+
       --proxy-trusted-headers string-array, $CODER_PROXY_TRUSTED_HEADERS
           Headers to trust for forwarding IP addresses. e.g. Cf-Connecting-Ip,
           True-Client-Ip, X-Forwarded-For.

cli/testdata/server-config.yaml.golden

@@ -6,6 +6,9 @@ networking:
   # "*.example.com".
   # (default: <unset>, type: url)
   wildcardAccessURL:
+  # Specifies the custom docs URL.
+  # (default: <unset>, type: url)
+  docsURL:
   # Specifies whether to redirect requests that do not match the access URL host.
   # (default: <unset>, type: bool)
   redirectToAccessURL: false