Add manage subscription page and nav

[tiffany-kobiton]

Summary

Manage your subscription page and add to navbar.

Related PRs, issues, or features (optional)

• N/A

Metadata

• [ ✅] Adds new file(s)
• [ ✅] Edits existing file(s)
• Removes file(s)

PR contributor checklist

• [ ✅] My PR follows the Kobiton Docs contributor guidelines, meaning:

  • My content contains correct spelling and grammar.
  • My directories and files are named and structured correctly.
  • My style and voice follows the Microsoft Style Guide.
  • I added all new pages to their section's nav.adoc file.
  • I removed all localizations (like en-us) from my URLs.

[tiffany-kobiton]

Blocked pending source verification from Gong/call notes or test account path.
Need to confirm purchase-flow entry point and account state before drafting.