Standardize Markdown headings; enforce MD003 with markdownlint

[jbampton]

Description

This PR adds a check / test with markdownlint to make sure we only use one type of Markdown heading in our docs.

https://github.com/DavidAnson/markdownlint/blob/main/doc/md003.md

Use .markdownlintignore for better command line support. Central location and easy to find and maintain

Types of changes

• Breaking change (fix or feature that would cause existing functionality to change)
• New feature (non-breaking change which adds functionality)
• Bug fix (non-breaking change which fixes an issue)
• Enhancement (improves an existing feature and functionality)
• Cleanup (Code refactoring and cleanup, that may add test cases)
• Build/CI
• Test (unit or integration test code)

Feature/Enhancement Scale or Bug Severity

Feature/Enhancement Scale

• Major
• Minor

Bug Severity

• BLOCKER
• Critical
• Major
• Minor
• Trivial

Screenshots (if appropriate):

How Has This Been Tested?

With pre-commit and visually checked

How did you try to break this feature and the system with this change?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 17.39%. Comparing base (a466750) to head (3fe877a).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##              main   #11688       +/-   ##
============================================
+ Coverage     3.62%   17.39%   +13.77%     
- Complexity       0    15285    +15285     
============================================
  Files          441     5889     +5448     
  Lines        37136   526184   +489048     
  Branches      6808    64242    +57434     
============================================
+ Hits          1346    91541    +90195     
- Misses       35626   424298   +388672     
- Partials       164    10345    +10181     

Flag	Coverage Δ
uitests	3.62% <ø> (ø)
unittests	18.44% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[DaanHoogland]

no code touched @harikrishna-patnala , I think you can merge at will.