Docs: Introduce Exploring the Catalog Section & Update Metrics/Leaderboard

[zihanKuang]

This PR fixes #617

Key Changes:

1. Updated Catalog Section: I've refreshed the images and descriptions for the Metrics and Leaderboard subsections under the original Catalog section.
2. Added "Exploring The Catalog" Chapter: This new section includes:
   • View (Top Performers)
   • Filtering and Sorting
   • Available Actions
3. Alert Box Titles: Standardized all alert boxes in this section to have meaningful titles, instead of generic ones like "Note."

Additional Considerations:

1. Article Title: I chose "Exploring The Catalog" for the article title, prioritizing brevity over options like "Exploring Meshery Catalog" or "Exploring Cloud Catalog"。
2. "Meshery Catalog" as a Proper Noun: I'm treating "Meshery Catalog" as a proper noun, and using lowercase "catalog" when referring to the general concept.
3. Potential for Best Practices/Q&A: I considered adding a "Best Practices" or "Q&A" section but couldn't think of relevant content. The current UI and page is quite straightforward.

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