docs: Alerts feature has no documentation page at all

## What's missing

This surfaced while investigating cubedevinc/cubejs-enterprise PR #14221 (`fix(alerts): paywall alert creation for non-paying tenants`), which fixed a bug that let free-plan tenants use Alerts (create/edit monitors, receive build-completed alert emails) when they shouldn't have been able to. Free-plan tenants will now notice alert creation blocked.

But the audit found something bigger: **the Alerts feature itself has no documentation anywhere in docs-mintlify.** Checked `docs/explore-analyze/notifications.mdx` (a different feature — scheduled-refresh email/Slack screenshots) and `admin/account-billing/budgets.mdx` (budget spend alerts, also different). No `alerts.mdx` or equivalent exists. The only occurrences of "Alerts" in the whole docs-mintlify tree are permission-scope names (`AlertsCreate`/`AlertsRead`/etc.) in the auto-generated `api-reference/api.yaml`.

## Why this needs a ticket instead of a docs PR

This isn't just "add a plan-availability note to the paywall change" — it's "the feature has no user-facing documentation to add that note to." Someone needs to scope what Alerts actually is (what it monitors, how to create/edit a monitor, what channels it notifies through, how it differs from the scheduled-refresh Notifications feature and from Budget alerts) before writing a page.

## Suggested scope

- Confirm with product/eng what "Alerts" covers today (monitor types, notification channels, admin vs. non-admin capabilities).
- New page (likely under `docs/explore-analyze/` or `admin/monitoring/`, alongside the related Notifications and Budgets pages) documenting the feature.
- Include plan availability (the free-plan paywall from #14221) as part of that new page, not as a standalone fix.

Filed as a GitHub issue rather than a Linear ticket because the Linear MCP connector isn't authorized in this session — recommend re-filing in Linear (or linking this issue there) once available.

Start by comparing docs/explore-analyze/notifications.mdx and admin/account-billing/budgets.mdx, then inspect the Alerts permission-scope entries in api-reference/api.yaml. Confirm with product and engineering what Alerts covers, including monitor types, notification channels, capabilities, and plan availability. Done means a new page documents the feature and distinguishes it from Notifications and Budget alerts.

Written by the indexing model from the issue text.