refactor: TableOfContents variants; deprecate LeftNavBar

[wackerow]

Extends what-is-ethereum:

• feat: /what-is-ethereum revamp #16042

Description

• Update TableOfContents variants to include left variant, matching style of LeftNavBar
• Deprecate LeftNavBar replacing with ToC variant="left"
• Rename beginner variant to card

Preview links

Variant: docs

Should have parity with production, e.g.,: https://ethereum.org/developers/docs/intro-to-ethereum/

• https://deploy-preview-16121--ethereumorg.netlify.app/en/developers/docs/intro-to-ethereum/

Variant: left (Replaces LeftNavBar)

Should be near-parity with production, e.g., https://ethereum.org/staking, but with slight adjustment to sticky-top distance, and now highlights section by scroll position

Without dropdown:

• https://deploy-preview-16121--ethereumorg.netlify.app/en/learn
• https://deploy-preview-16121--ethereumorg.netlify.app/en/what-are-apps

With dropdown:

• https://deploy-preview-16121--ethereumorg.netlify.app/en/staking
• https://deploy-preview-16121--ethereumorg.netlify.app/en/nft

Variant: card (renamed from beginner, new in #16042)

• https://deploy-preview-16121--ethereumorg.netlify.app/en/what-is-ethereum

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[wackerow]

Also refactored translatathon page to use TableOfContents component, updating to add a conditional hiding the "Apply" section if the deadline has passed. Added a DRY date utility helper to return a boolean for when a date has been reached.