Connect Zcash real-world playbook journey

[shensz2017]

/claim #1555

Summary:

• Add the /zcash-use-case slug and a linear journey overview to site/Zcash_Use_Cases/About.md.
• Connect the six real-world Zcash playbooks into a beginner-to-advanced path.
• Fix broken next-step and related links across the existing playbook pages.

Verification:

• Compared branch against main; diff is scoped to the real-world playbook pages.
• Checked internal links across the updated playbook files against existing site/ markdown paths.

Closes #1555