CI/DOC: more detailed documentation on how CI works

Signed-off-by: Raphael Defosseux <raphael.defosseux@eurecom.fr>

ci-scripts/doc/vm_based_simulator_check_build.md

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+<table style="border-collapse: collapse; border: none;">
+  <tr style="border-collapse: collapse; border: none;">
+    <td style="border-collapse: collapse; border: none;">
+      <a href="http://www.openairinterface.org/">
+         <img src="../../doc/images/oai_final_logo.png" alt="" border=3 height=50 width=150>
+         </img>
+      </a>
+    </td>
+    <td style="border-collapse: collapse; border: none; vertical-align: center;">
+      <b><font size = "5">OAI CI Virtual-Machine-based: checking the build result</font></b>
+    </td>
+  </tr>
+</table>
+
+## Table of Contents ##
+
+1.  [Introduction](#1-introduction)
+2.  [Detailed Description](#2-detailed-description)
+
+# 1. Introduction #
+
+Function is called:
+
+- when building in foreground
+- when waiting for the background build process is finished
+
+# 2. Detailed Description #
+
+Source file concerned: `ci-scripts/waitBuildOnVM.sh`
+
+## 2.1. check_on_vm_build function ##
+
+*  Retrieve the build log files from the VM instance `ubuntu@$VM_IP_ADDR:/home/ubuntu/tmp/cmake_targets/log/*.txt`
+*  and copy them locally in the workspace at $ARCHIVES_LOC
+*  List all log files that match the pattern. Each should have
+   *  the `Built target` pattern (the library/executable SHALL link)
+*  The number of patterned log files SHALL match $NB_PATTERN_FILES defined in `ci-scripts/oai-ci-vm-tool` script for the variant
+
+---
+
+Next step: [how to test a function](vm_based_simulator_test)
+
+You can also go back to the [CI dev main page](./ci_dev_home)
+