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 17.55%. Comparing base (b106d6e) to head (de37267).
⚠️ Report is 13 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #11688      +/-   ##
============================================
- Coverage     17.56%   17.55%   -0.01%     
+ Complexity    15517    15515       -2     
============================================
  Files          5906     5906              
  Lines        528377   528377              
  Branches      64528    64528              
============================================
- Hits          92783    92775       -8     
- Misses       425162   425170       +8     
  Partials      10432    10432              

Flag	Coverage Δ
uitests	3.59% <ø> (ø)
unittests	18.62% <ø> (-0.01%)	⬇️

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[DaanHoogland]

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

[DaanHoogland]

ping @harikrishna-patnala ?

[vishesh92]

lgtm