Quality-of-life markdownlint YAML improvements

What does this MR do?

Revises .markdownlint.yml with these goals:

• Put the settings in alphabetical order, with two sensible exceptions: default: true should go first, and proper-names: is long so let's move it last
• Add tidy inline comments with the rule number. Markdownlint updates typically reference rules by their number, and future us will be grateful that we can grep for the rule number and know whether or not a deprecation affects us.
• Add URL to full rule list. Not everyone knows where this is, and because we touch this file so rarely, it's helpful to not have to thrash.

Related issues

Author's checklist

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Reviewer's checklist

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