Document optional public profile for users and vendors

[kewitz]

Reflects changes from opencollective-frontend commit 988f0658c8:

• Users can now disable/enable their public profile via Dashboard > Settings > Info
• Vendors can optionally enable a contributor profile to showcase platform activity
• Clarifies distinction between disabled profiles and private organizations

Updates:

• getting-started/editing-your-profile-page.md: Added Public Profile Visibility section
• fiscal-hosts/managing-your-collectives/vendors.md: Added Contributor Profile option
• organizations/private-organizations.md: Clarified differences between account types