Add a markdownlint-cli step for the book #72
Description
Follow-up to #61, similar to what we have at the sway repo: https://github.com/FuelLabs/sway/blob/master/.github/workflows/markdown-lint.yml
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Follow-up to #61, similar to what we have at the sway repo: https://github.com/FuelLabs/sway/blob/master/.github/workflows/markdown-lint.yml