Update landing page layout

[scott-klaytn]

Proposed changes

• Modify the grid layout for better affordance
• Move some entries (link to external sites) to the footer area to streamline the landing page

Types of changes

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• Minor Issues and Typos
• Major Content Contribution
• Others

Checklist

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• I have read the CONTRIBUTING GUIDELINES
• I have read the CLA and signed by comment I have read the CLA Document and I hereby sign the CLA in first time contribute
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged and published in downstream modules

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