Avoid URL checks inside code blocks

[lukicdarkoo]

The test_references.py test doesn't consider whether the link is found in a code block or not:

webots/docs/tests/test_references.py

Line 107 in 2500811

for m in re.finditer(r"[^!]\[(.*?)\]\(([^\)]+)\)", content):

We should be able to write anything in code blocks, without restrictions. It might be possible that some other tests are also checking the content in the code blocks.

The problem first occurred in:
https://github.com/cyberbotics/webots/pull/2279/checks?check_run_id=2775845374