Update MainDashboardButton documentation

[afercia]

Description

To my understanding, the edit-site MainDashboardButton was removed in #45149 so that this Slot is only used in the post editor.

However, the documentation still refers to the post editor and site editor and contains example code for the site editor. See:
https://github.com/WordPress/gutenberg/blob/b8c177d255cf0b8bab7afa82bfbd40f93e69277f/docs/reference-guides/slotfills/main-dashboard-button.md

Cc @youknowriad

Step-by-step reproduction instructions

N/A

Screenshots, screen recording, code snippet

No response

Environment info

No response

Please confirm that you have searched existing issues in the repo.

Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

Yes