Alerts - Ability to add description field with markdown to the Alert

### Problem Statement

This one is specific to Slack:
What's still missing is option to have title and separate longer description that could include 'clickable' links that would be also shared in alert.

We would like to for example include more info in description around what's causing the issue and link to runbook on how to possibly address specific alert.

Example of alert that could use more context information:
https://x.sentry.io/alerts/rules/details/x/

Currently we can only define title as metadata.

What would we need is just one more field for description, ideally allowing markdown formatting so we can include links etc.
Why this is needed? We have critical alerts which are often acted upon by our support team first. Support team might not be experts in specific app but can try to help out quickly if they can get promptly more context for given alert.

### Solution Brainstorm

What would we need is just one more field for description, ideally allowing markdown formatting so we can include links etc.

### Product Area

Alerts

┆Issue is synchronized with this [Jira Improvement](https://getsentry.atlassian.net/browse/FEEDBACK-2291) by [Unito](https://www.unito.io)