Improve ASA overview and add asset metadata page

[Ganainmtech]

This PR introduces an Asset Metadata overview page and improves the structure and clarity of the ASA overview documentation.

Changes include:

• Added a new Asset Metadata page describing ASA metadata fields, off-chain metadata usage, and relevant ARC standards
• Improved the structure and readability of the Mutable Asset Parameters section
• Clarified the roles of the manager, reserve, freeze, and clawback addresses
• Consolidated the irreversible empty-address behaviour into a single note for clarity
• Removed the outdated known-assets page