Add a lint check that every explainer has the required sections #1118

Where: a new scripts/check_explainer_structure.py (or an extension of scripts/check_explainer_count.py), Makefile, .github/workflows/lint.yml, .pre-commit-config.yaml, tests/.

Current gap: CONTRIBUTING.md section 8 lists the sections a good explainer has (definition, why it matters, concrete example, detection code, limitations, related concepts, further reading) and the explainer template adds a footer line. Nothing checks it. Four explainers have no Limitations section (proxy-variables, sampling-bias, neural-networks, automation-bias), and the heading wording varies ("Why It Matters" against "Why This Matters"), so a reader cannot rely on the structure and a contributor can merge an explainer that skips a required part.

Suggested approach: define the required sections with their accepted heading variants (a small table in the script), check each explainers/*.md for them in order, for a python fenced block, for at least two Further Reading entries and for the footer line, and print the file and what is missing. Start with a list of known exceptions for the four pages above (to be removed when the Limitations sections are added) so the check can land first.

Done when: make lint and the lint workflow fail on an explainer missing a required section, the exceptions list is empty once the four pages are fixed, and the check has tests.