Fix HTML validation issues (W3C validator) (#964)

Closes #963.

- redundant `role="navigation"` on nav elements
- redundant `type="text/javascript"` on script tags
- misplaced mermaid script tag

Note that [running the validator on the deploy preview](https://validator.w3.org/nu/?doc=https%3A%2F%2Fdeploy-preview-964--just-the-docs.netlify.app%2F) has no errors now!