ci(doc-linter): anchor .. code-block:: detection + drop debug print

Addresses Copilot review on PR #2023:

1. .. code-block:: tracking was triggered by a plain substring check, which
   matched mid-line occurrences too. In MD prose like ``.. code-block::``
   (docs/documentation.md:222) this set in_rst_codeblock=True spuriously and
   could suppress line-length checks downstream. Replace with a leading-whitespace-
   anchored regex and gate on file_ext in ('.rst', '.txt') or an open {eval-rst}
   MyST fence so the directive opener is only recognized where it can actually
   occur.

2. print('start') in main() was leftover debug noise — remove it.

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