Add ZecMap wiki page

[gshaowei6]

Addresses #1653 and #1654.

Summary:

• adds a ZecMap wiki page under site/Using_Zcash
• explains merchant discovery, business submissions, verification, contributor maintenance, key features, rewards, and future development
• keeps the page practical for users who want to find or add ZEC-accepting businesses

Verification:

• checked the requested output path and file name
• checked the page covers every bullet in the issue description
• reviewed markdown locally for structure and link formatting