Add markdownlint for documentation formatting consistency

[bmish]

Add popular tool markdownlint-cli which runs markdownlint via a package.json script for documentation formatting consistency and best practices.

The vast majority of these changes were autofixed.

Summary of the main changes:

• Each file should have only one top-level header
• Header sizes should decrement one level at a time
• No consecutive blank lines
• All code fences should specify their language
• Lists should use the same type of bullet point

There's a good markdownlint VSCode extension which highlights and autofixes these issues as well: https://marketplace.visualstudio.com/items?itemName=DavidAnson.vscode-markdownlint

[codecov]

Codecov Report

Merging #3367 (ae34341) into master (11306e1) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3367   +/-   ##
=======================================
  Coverage   97.54%   97.54%           
=======================================
  Files         123      123           
  Lines        8873     8873           
  Branches     3244     3244           
=======================================
  Hits         8655     8655           
  Misses        218      218           

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[ljharb]

Thanks for bearing with me!