Improve documentation for protecodeExecuteScan (#3076)

* Improve documentation for protecodeExecuteScan

protecodeExecuteScan expects that the docker config file is named
'config.json'.

The dockerConfigJSON parameter is used by protecodeExecuteScan to
retrieve the directory only (for environment variable DOCKER_CONFIG).

* Also improve documentation that is generated

* No relative references in generated documentation.

* Less complex documentation

* Remove irrelevant empty line

* Improvements after review

* Add blank line after code block

* Handle registry URL example as code

Markdown format checker does not like raw URLs.

Co-authored-by: Sven Merk <33895725+nevskrem@users.noreply.github.com>

documentation/docs/steps/protecodeExecuteScan.md

@@ -8,7 +8,25 @@
 1. Look up your Group ID using REST API via `curl -u <username> "https://<protecode host>/api/groups/"`.
 
 If the image is on a protected registry you can provide a Docker `config.json` file containing the credential information for the registry.
-You can create it like explained in the Docker Success Center in the article about [how to generate a new auth in the config.json file](https://success.docker.com/article/generate-new-auth-in-config-json-file).
+You can either use `docker login` (see the [Docker documentation](https://docs.docker.com/engine/reference/commandline/login/) for details) or you can create the file manually using the following script.
+
+```shell
+#!/bin/bash
+auth=$(echo -n "$USER:$PASSWORD" | base64 -w0)
+cat <<EOF > config.json
+{
+    "auths": {
+        "$REGISTRY": {
+            "auth": "$auth"
+        }
+    }
+}
+EOF
+```
+
+`REGISTRY` is the URL of the protected registry (Example: `https://index.docker.io/v1`).
+
+Attention: If you reference the file in --dockerConfigJSON or upload the file to the Jenkins credential store, the file has to be named `config.json`.
 
 ## ${docGenParameters}