Add social cards

[0xlukem]

Description

This PR adds a social description for the homepage.
This PR needs to be merged with this one: papermoonio/moonbeam-mkdocs#300

Checklist

• Added a label 🏷️ to this PR
• Ran my changes through Grammarly
• Added a disclaimer if required
• If pages were moved, opened a corresponding PR in moonbeam-mkdocs to update redirects

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Translations

Does this PR update a page that also exists on the Chinese docs site? See mapping to confirm.

• Yes
• No

If Yes, complete the following:

• Opened a PR on the Chinese docs repo for the corresponding page
• Updated images, snippets, or variables if they were moved, renamed, or deleted

Link to the corresponding CN docs PR: <INSERT_LINK>

[Copilot]

Pull request overview

This PR adds a social_description metadata field to the homepage to support social media card generation. The new field provides a concise, marketing-focused description of Moonbeam that emphasizes its dual nature as an Ethereum-compatible platform on Polkadot. This change depends on a corresponding PR in the moonbeam-mkdocs repository (#300) that likely implements the template logic to utilize this new field for Open Graph meta tags.

Changes:

• Added social_description front matter field to the homepage with a concise description suitable for social media cards

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