Remove cppcheck from CI, add to tools/

We don't enforce cppcheck through the CI, although it's there since
years. It runs on Ubuntu 18/20, so it's old. For folks, it's likely not
discoverable on how to run it locally. Let's make a fresh start.

This removes cppcheck from all CI-related code. Instead, it adds it
under tools/cppcheck/, including documentation on how to run it locally,
bare-matel or in docker.

Signed-off-by: Robert Schmidt <robert.schmidt@openairinterface.org>

tools/cppcheck/README.md

@@ -0,0 +1,28 @@
+<!-- SPDX-License-Identifier: CC-BY-4.0 -->
+
+# Static code analysis
+
+How to run static code analysis tool(s).
+
+[[_TOC_]]
+
+## cppcheck Overview
+
+This can run a (dockerized) cppcheck static code analysis. The docker-compose
+file represents the status as run as in the CI until it's removal (because it
+was not enforced). It is now here so that people could use it to find bugs (and
+use the tool more easily).
+
+## cppcheck Usage
+
+From the openairinterface5g root, simply run
+
+    ./tools/cppcheck/run-cppcheck.sh
+
+This will run cppcheck with options listed in this file, assuming you have it
+installed. Alternatively, you can run the CI version as of Ubuntu 20 with
+
+    cd tools/cppcheck && docker compose up --no-log-prefix
+
+You can upgrade to a newer version by modifying the `FROM` line in the
+docker-compose file.