Add markdownlint Rules to Enforce Specific Formatting

[mcdonnnj]

🗣 Description

This PR adds new rules for MD035 and MD046 to our markdownlint pre-commit configuration.

💭 Motivation and context

These are the last two rules that default to consistent. We should prefer a specific style that is consistent across all of our projects instead of only ensuring that things are done consistently across a single project.

🧪 Testing

Automatic testing passes. I confirmed that the rules are enforced with local testing:

# This is the main heading #

## This is a sub-heading with a horizontal rule ##

***

## This is a sub-heading with an indent style code block ##

    # #!/usr/bin/env bash
    # echo "Hello, World!"

$ pre-commit run markdownlint --files mdl_test.md
markdownlint.............................................................Failed
- hook id: markdownlint
- exit code: 1

mdl_test.md:5 MD035/hr-style Horizontal rule style [Expected: ---; Actual: ***]
mdl_test.md:9 MD046/code-block-style Code block style [Expected: fenced; Actual: indented]

✅ Checklist

• [x] This PR has an informative and human-readable title.
• [x] Changes are limited to a single goal - eschew scope creep!
• [x] All relevant type-of-change labels have been added.
• [x] I have read the CONTRIBUTING document.
• [x] These code changes follow cisagov code standards.
• [x] Tests have been added and/or modified to cover the changes in this PR.
• [x] All new and existing tests pass.