docs: document sending email notifications to user groups

[mngr]

Description

Documents that dashboard scheduled-refresh email notifications can now be sent to user groups alongside individual users.

Changes to docs-mintlify/docs/explore-analyze/notifications.mdx:

• Delivery channel — the Email bullet now notes that the recipient picker is grouped into Users and User groups sections, so you can add individual workspace users, entire user groups, or a mix of both.
• Email notifications — new Recipients subsection describing the two picker sections and the behaviour that matters at send time:
  • A group is expanded to its current members each time the notification is sent, so membership changes take effect on the next run without editing the schedule.
  • The User groups section only appears when the workspace has at least one user group.
  • A recipient selected individually who is also a member of a selected group is emailed only once.
• Added a [ref-user-groups] cross-link to the user groups page.

Notes

Docs-only change. I did not run the full Mintlify build locally; the link targets were verified against the existing pages (user-groups.mdx, matching the [ref-roles] link style already used on this page).

[mngr]

Superseded by #11301 — recreated from a branch on the upstream repo so the required review-with-tracking check can run (it can't fetch an OIDC token on fork PRs). Same content, rebased onto current master.