[Auditbeat] Cherry-pick #9670 to 6.x: Enable system module docs (#9716)

Cherry-pick of PR #9670 to 6.x branch. Original message: 

With elastic/docs#512 merged we can enable links to `x-pack/auditbeat/docs` to have the system module included in the documentation.

auditbeat/docs/modules_list.asciidoc

@@ -4,9 +4,11 @@ This file is generated! See scripts/docs_collector.py
 
   * <<{beatname_lc}-module-auditd,Auditd>>
   * <<{beatname_lc}-module-file_integrity,File Integrity>>
+  * <<{beatname_lc}-module-system,System>>
 
 
 --
 
 include::./modules/auditd.asciidoc[]
 include::./modules/file_integrity.asciidoc[]
+include::../../x-pack/auditbeat/docs/modules/system.asciidoc[]

auditbeat/scripts/docs_collector.py

@@ -153,20 +153,10 @@ def collect(base_paths):
 
     module_list_output = generated_note
     for m, title in sorted(six.iteritems(modules_list)):
-        if m == "system":
-            # This is a very crude way of excluding references to X-Pack docs from
-            # the build until the docs build is updated with the new folder.
-            # TODO: Remove at the earliest possible time.
-            continue
         module_list_output += "  * <<{beatname_lc}-module-" + m + "," + title + ">>\n"
 
     module_list_output += "\n\n--\n\n"
     for module_name, module_path in sorted(six.iteritems(module_dirs)):
-        if module_name == "system":
-            # This is a very crude way of excluding references to X-Pack docs from
-            # the build until the docs build is updated with the new folder.
-            # TODO: Remove at the earliest possible time.
-            continue
         rel_path_to_module_docs = os.path.relpath(module_docs_path(module_path), docs_path)
         module_list_output += "include::" + \
             os.path.join(rel_path_to_module_docs, "modules", module_name + ".asciidoc") + "[]\n"