Markdownlint: Move required heading structure into TOP004 rule directly

[MaoShizhong]

Because

We use different required heading structures for lesson and project files which are currently being handled by separate config files that extend the base markdownlint config. Because of this, the VSCode extension doesn't pick up any TOP004 errors. Because of this, people sometimes make unnecessary "fixes" because they ran the wrong lint script (e.g. lesson script for a project file).

By moving the heading structure definitions into the rule itself, we can make the rule determine which heading structure to use based on the file name, eliminating the need for separate configs and scripts.

This PR

• Moves heading structure definitions into the TOP004 rule file
• Deletes previous lesson/project extending config files
• Removes redundant functions (carryovers from original MD043 rule) like ellipsify and handleCase
• Provides better error message for missing heading error, otherwise no message would show for VSCode extension errors (the context field only comes up in the terminal report)
• Adds/amends TOP004 test files to demonstrate different errors, including lesson/project structure difference
• Replaces config-dependent linting scripts with simple scripts for linting, and fixing
• Amends repo contributing guide and relevant custom rule doc files to account for changes
• Removes custom config from markdownlint GH actions (default is .markdownlint-cli2.jsonc)

Issue

N/A

Additional information

Sorry for the markdownlint bomb recently, Eric!

Pull Request Requirements

• I have thoroughly read and understand The Odin Project curriculum contributing guide
• The title of this PR follows the location of change: brief description of change format, e.g. Intro to HTML and CSS lesson: Fix link text
• The Because section summarizes the reason for this PR
• The This PR section has a bullet point list describing the changes in this PR
• If this PR addresses an open issue, it is linked in the Issue section
• If any lesson files are included in this PR, they have been previewed with the Markdown preview tool to ensure it is formatted correctly
• If any lesson files are included in this PR, they follow the Layout Style Guide