fix(colab): Add linter for malformed lists in generated notebooks - #3947

Fixes #3946

Description

Some list structures in Sphinx-Gallery tutorial sources render correctly in the HTML documentation but become malformed after conversion to Markdown in the generated Google Colab notebooks.

This PR adds a repository-local TUTORIAL_MARKUP Lintrunner check that detects the two source patterns responsible for the problems documented in #3946:

• indented lists that begin without a separating blank narrative line; and
• list continuation lines that are not indented beneath their list item.

The checker only examines Sphinx-Gallery narrative content from module docstrings and gallery comment blocks. It accounts for valid nested lists, reStructuredText directives, and list tables to avoid reporting those as violations.

Validation strategy

This draft is being built in two commits so the Lintrunner behavior can be verified directly in GitHub Actions:

1. Add and register the linter.
   The expected CI result is 14 diagnostics across exactly the 10 tutorials documented in [BUG] - Incorrect list rendering in Google Colab tutorials #3946, with no additional tutorial files reported.
2. Fix the detected violations.
   The subsequent CI run should pass with no TUTORIAL_MARKUP diagnostics.

Local validation

python3 -m unittest discover -s tools/linter/tests -p 'test_*.py'
lintrunner --all-files --take TUTORIAL_MARKUP

The unit test suite currently passes all eight tests. The complete linter scan reports 14 diagnostics across the 10 affected tutorial files and reports no other tutorial files.

Checklist

• The issue that is being fixed is referred in the description (see above "Fixes #ISSUE_NUMBER")
• Only one issue is addressed in this pull request
• Labels from the issue that this PR is fixing are added to this pull request
• No unnecessary issues are included into this pull request.