Markdown linting: low hanging fruit

[SeanKilleen]

Summary of the changes (Less than 80 chars)

• A continuation of the markdown linting work started in Add Markdownlint to CI (and ignore all rules for now) #6842.
  • That PR should be merged first.

This gets some of the easiest items out of the way, to clear the room for the others.

Resolves:

• MD009 (trailing whitespace)
• MD012 (multiple line blanks)
• MD018 (extra spaces in heading)
• MD022 (blank lines around headings)
• MD031 (blanks around code fences)
• MD032 (blanks around lists)
• MD034 (bare URLs)

Supports #5460.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (01daf1e) 69.52% compared to head (f80c606) 73.04%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6843      +/-   ##
==========================================
+ Coverage   69.52%   73.04%   +3.52%     
==========================================
  Files        2481     2485       +4     
  Lines      125456   125526      +70     
==========================================
+ Hits        87219    91691    +4472     
+ Misses      38237    33835    -4402     

Flag	Coverage Δ
unittests	73.04% <ø> (+3.52%)	⬆️

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[SeanKilleen]

@michaelstaib this workflow is ready for approval.

@glen-84 feel free to review. You may want to go commit by commit as I address each rule in a commit. I kept this one simple, addressing mostly items around whitespace. I've incorporated the changes from the previously merged PR that adds the markdown lint support.

[michaelstaib]

See @glen-84 comments

[SeanKilleen]

@glen-84 @michaelstaib fixed.

[michaelstaib]

Thank you @SeanKilleen and @glen-84