This PR adds a markdownlint configuration and workflow to enforce consistent markdown style.

• resolves Enforce markdown style with a linter workflow #398

It touches a lot of files because it also brings our existing markdown into compliance with the linter as configured.

CoPilot Summary

This pull request includes several updates to Markdown linting workflows, configuration, and various Markdown files to improve consistency and readability. The most important changes include the addition of a GitHub Actions workflow for linting Markdown files, updates to the Markdown linting configuration, and numerous formatting adjustments across multiple Markdown files.

Workflow and Configuration Updates:

• Added a new GitHub Actions workflow for linting Markdown files on push and pull request events in .github/workflows/lint_md_changes.yml.
• Updated .markdownlint.yml configuration to disable certain noisy rules and enforce specific styles for unordered lists and emphasis.

Formatting Adjustments:

• Replaced underscores with asterisks for emphasis in README.md, docs/_includes/helping_out.md, and docs/about/changelog.md to align with the updated linting rules. [1] [2] [3]
• Changed plain URLs to angle-bracketed URLs in README.md for better Markdown compliance. [1] [2] [3]
• Removed unnecessary blank lines in various Markdown files to improve readability and comply with linting rules. [1] [2] [3] [4]

These changes enhance the project's Markdown documentation quality and ensure consistency across the repository.