doc: add internal documentation

This adds documentation that is meant as internal only. As first entry
the `--inspect-brk-node` flag got documented here.

PR-URL: #26665
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

doc/guides/internal/readme.md

@@ -0,0 +1,19 @@
+These flags are for Node.js core development usage only. Do not use these flags
+in your own applications. These flags are not subjected to semantic versioning
+rules. The core developers may remove these flags in any version of Node.js.
+
+# Internal documentation of Node.js
+
+## CLI
+
+### Flags
+
+#### `--inspect-brk-node[=[host:]port]`
+
+<!-- YAML
+added: v7.6.0
+-->
+
+Activate inspector on `host:port` and break at start of the first internal
+JavaScript script executed when the inspector is available.
+Default `host:port` is `127.0.0.1:9229`.