docs: remove account and zone limits from Organizations page

[KaydeeDee]

This PR updates the Organizations documentation with two changes:

Changes:

1. Organizations overview page (src/content/docs/fundamentals/organizations/index.mdx)

   • Removed the account and zone limits from the overview
   • Changed from: "Each user can create one organization, and each organization supports up to 500 accounts and 500 zones."
   • Changed to: "Each user can create one organization."

2. Organizations changelog (src/content/changelog/fundamentals/2026-04-06-organizations-public-beta.mdx)

   • Updated zone limit from 500 to 5000
   • Changed from: "Each Organization supports up to 500 accounts and 500 zones"
   • Changed to: "Each Organization supports up to 500 accounts and 5000 zones"