refactor: mark toc in loaded MDX as non-optional

[Josh-Cena]

Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.
• If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• If this is a new API or substantial change: the PR has an accompanying issue (closes #0000) and the maintainers have approved on my working plan.

Motivation

I got confused when writing the type declarations. The loaded MDX actually always has a toc variable available, even if it's just an empty array. I've updated the type defs and made relevant refactors.

Made a test to make this clearer.

Test Plan

Test links

Deploy preview: https://deploy-preview-_____--docusaurus-2.netlify.app/

Related issues/PRs

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[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🟠 77	🟢 100	🟢 100	🟢 100	🟢 90	Report
/docs/installation	🟠 87	🟢 99	🟢 100	🟢 100	🟢 90	Report

[github-actions]

Size Change: 0 B

Total Size: 797 kB

ℹ️ View Unchanged

Filename	Size
website/.docusaurus/globalData.json	52.5 kB
website/build/assets/css/styles.********.css	106 kB
website/build/assets/js/main.********.js	600 kB
website/build/index.html	38.9 kB

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