Set up markdownlint as GitHub Action for automated linting

[gwtaylor]

Summary

Add automated markdownlint checking via GitHub Actions to ensure consistent Markdown formatting across all PRs.

Background

This suggestion came from @egrace479's review of PR #56 (see comment) where she noted that manual linting could be automated.

Proposed Implementation

• Add a GitHub Action workflow that runs markdownlint on all Markdown files
• Use the existing .markdownlint.json configuration
• Run on pull requests and pushes to main/dev branches
• Consider using markdownlint-cli2 for better performance and configuration support

Benefits

• Catch formatting issues automatically before merge
• Reduce manual review overhead for formatting concerns
• Ensure consistent formatting across all contributors
• Align with the contributing guidelines established in PR Add CONTRIBUTING.md with project standards and guidelines #56

Acceptance Criteria

• GitHub Action workflow file created
• Workflow runs markdownlint on PR creation and updates
• Uses project's .markdownlint.json configuration
• Provides clear error messages for contributors
• Documents the automation in CONTRIBUTING.md

Related

• PR Add CONTRIBUTING.md with project standards and guidelines #56: Add CONTRIBUTING.md with project standards and guidelines
• Review comment: Add CONTRIBUTING.md with project standards and guidelines #56 (comment)

[egrace479]

To be implemented in the Collab Guide template repo (see issue).

[egrace479]

This was completed in the Collab Guide, need to cherry-pick commit b10d3be.