Hoist markdownlint to root

What does this MR do?

This hoists markdownlint tooling/configuration to the root.

This may be easier to review commit-by-commit.

Part of #2034 (closed).

Commits

Fix markdown lints

The next commit will enable linting for these files, so fix the lints first.

Hoist markdownlint to root

We're sticking with markdownlint-cli2, since it is a bit more flexible for our needs than markdownlint-cli.

See https://dlaa.me/blog/post/markdownlintcli2 for more information about the design goals of markdownlint-cli2.

Part of #2034 (closed).

Use lint: prefix for markdownlint script

This is to follow the convention used for eslint and prettier.

Screenshots or screen recordings

N/A

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• GitLab: mr_url
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