[Docs] Add video page creation guidelines to the Contributing Guide

[zihanKuang]

Description

Meshery Docs should include clear instructions on how to create individual video pages. Currently, contributors may not know:

• Where and how to add a new video page.
• Which frontmatter fields or shortcodes are required.
• How to maintain consistency across different types of documentation (e.g., guide, task, event, video, blog).

Checklist

• A new section titled Creating a Video Page is added to the Contributing Guide.
• The section includes:
  • Required frontmatter fields for video pages (e.g., title, video_link, duration, tags, etc.).
  • How to use the custom shortcode (if applicable) to embed videos.
  • Guidelines on where to place video markdown files within the repo.