Use mdformat (instead of Markdownlint)

[jpmckinney]

https://github.com/executablebooks/mdformat will actually change your files, whereas Markdownlint will only complain.

Need to update https://ocds-standard-development-handbook.readthedocs.io/en/latest/standard/technical/lint.html

[jpmckinney]

Transferring back here. I forgot other repos all use RST, so this is really just for standard documentation. The issue for other repositories is open-contracting/software-development-handbook#35

[jpmckinney]

Hmm, mdformat:

• Prefers - for bullets to *
• Backslash escapes literal [ and ] characters
• Prefers YAML frontmatter to shorthand options

Its behavior is not configurable.

I don't like the backslash escapes (not how people write naturally), and the YAML frontmatter is just very verbose. We'll stick to Markdownlint.