Add skills intro video to documentation

[SannidhyaSah]

This PR adds an embedded YouTube intro video to the Skills documentation page.

Changes:

• Renamed docs/features/skills.md to docs/features/skills.mdx to support JSX syntax
• Added responsive YouTube iframe embed for the skills intro video (https://youtube.com/shorts/cK2CBU54bhw)
• Video appears after the opening paragraph with consistent styling matching other documentation pages

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Important

Adds a responsive YouTube intro video to skills.mdx, requiring a file extension change to support JSX syntax.

• Documentation:
  • Renamed docs/features/skills.md to docs/features/skills.mdx to support JSX syntax.
  • Added a responsive YouTube iframe embed for the skills intro video in skills.mdx.
  • Video is placed after the opening paragraph with styling consistent with other documentation pages.

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Review complete. No issues found.

The PR correctly renames skills.md to skills.mdx to enable JSX syntax and adds a responsive YouTube embed that follows the established patterns used in other documentation pages (checkpoints.mdx, auto-approving-actions.mdx, etc.).

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