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"

Preview links

Variant: docs

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

Variant: left (Replaces LeftNavBar)

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: beginner (new in #16042)

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

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