[Uptime] Clarify the difference between uptime monitors and uptime alerts in the docs

### Problem Statement

Customer feedback regarding Uptime monitor creation and how we point out the notification creation [here](https://docs.sentry.io/product/alerts/uptime-monitoring/#notifications):

> In the area where I create alerts there is an option for "Uptime Monitoring" which does not create alerts - and instead it creates an issue, so you have to make an Issue Alert. - it is that which is not clear in the documentation.
>
> As I go to make an alert and I see the option for uptime monitoring my assumption is that this will give me an alert when downtime is detected but it does not do that.
> It is not clear that it does not do that.
> 
>
> All it does it make an issue, which is not an alert, and setting this up in the alerts area is misleading.
> If I look at the other options available here all other options allow the sending of notifications via an action
> 
>
> It is only the uptime monitoring that does not have this option:
> 
>
> It is here where I expected to be able to set the action to send to our Teams channel.
> But that isn't the case for Uptime Monitoring - only via a separate issue alert.

### Solution Brainstorm

Clarify somewhere in the UI or the docs that the Uptime Monitor creation will not create notifications. Alternatively, provide the option to choose a notification action during uptime monitor creation too.

### Product Area

Crons

Start with the Uptime Monitoring documentation, especially the Notifications section linked in the issue, and compare its explanation with the alert-creation flow described in the screenshots. Done means clearly distinguishing monitor-created issues from notifications sent through a separate Issue Alert, though the issue also leaves open whether the UI should change.

Written by the indexing model from the issue text.