🚀 Feature: Use `@eslint/markdown` instead of markdownlint

[JoshuaKGoldberg]

Bug Report Checklist

• I have pulled the latest main branch of the repository.
• I have searched for related issues and found none that matched my issue.

Overview

As of late 2024, ESLint now supports linting Markdown natively: https://eslint.org/blog/2024/10/eslint-json-markdown-support. There are two reasons why I'd like to go with @eslint/markdown instead of Markdownlint:

• Long-term, there's no real structural reason not to unify the linters - and the ESLint project is well-positioned to be a general web dev languages linter
• Removing Markdownlint means reducing the number of root-level files by two: removing .markdownlint.json and .markdownlintignore

Additional Info

This is currently blocked on @eslint/markdown & its ecosystem adding in all the important rules that CTA uses from Markdownlint today. That includes the https://github.com/JoshuaKGoldberg/sentences-per-line contributed Markdownlint rule: eslint/markdown#317 -> I'll have to make that as a community plugin.

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