Runbook Link in Alert Definitions

### Problem Statement

## Problem Statement
Currently when our alerts page us through our 3rd party alerting tool, they provide the title and description of the alert, but any additional context is lost. What would be really helpful is the ability to add a link to an Alert definition that we could use to point at a high context runbook written specifically for that alert.

## Solution Brainstorm
An extra text field, possibly in the Establish Ownership section of the Alert definition, that supports a generic link to any URL. Our runbooks are currently hosted in Notion, but I can imagine some folk have them in google docs, confluence, etc. That field would be included on alert payloads that are sent to connected integrations like Incident.io, PagerDuty, etc.

I've opened up this additional issue to follow up on https://github.com/getsentry/sentry/issues/111808, which I'm not able to comment on or re-open! Sorry for the duplication, additional context about what's coming next in the Alert UI would be awesome!

## Specific questions around the new UI
* [sentaur-athena](https://github.com/sentaur-athena) mentioned that developers will be able to ad a link to their alert, but it sounds like that link would have to be to a specific Sentry URL, not a generic one. Is that the case? Would it be possible to add support for runbooks outside of Sentry? (it would suck to have to duplicate runbooks across a bunch of different providers)
* Her comment also mentioned that the alert payload sent when the alert trigger would include a link to the Alert, but not the new developer-added Link. Is there any chance it could be included in the alert payload that is sent to third party alerting providers? Slack, PagerDuty, Incident.io, etc. would all be able to parse it out and include it in the notification that gets to a production operator, so they don't have to click around a bunch of different UIs when they really want to quickly evaluate the severity and context of the outage.

Thanks for any additional context here!! 🙇

## Product Area
Alerts

### Solution Brainstorm

_No response_

### Product Area

Unknown

Start by reviewing the related issue 111808 and the current Alerts product context. Trace how alert definitions are represented and how alert payloads reach third-party integrations such as Incident.io, PagerDuty, and Slack; done means a generic runbook URL can be defined and is included in those payloads.

Written by the indexing model from the issue text.