`sl-alert`: move description from docs to component (#947)

* move component description from docs to component

* use component description in docs

* add style for lead sentence, keep bc for not updated components

* run `npm prettier`

* render summary as part of the header

* add `title` as another custom tag for JSDoc

* render components title in header

docs/components/alert.md

@@ -2,8 +2,6 @@
 
 [component-header:sl-alert]
 
-Alerts are used to display important messages inline or as toast notifications.
-
 ```html preview
 <sl-alert open>
   <sl-icon slot="icon" name="info-circle"></sl-icon>

src/components/alert/alert.ts

@@ -15,6 +15,9 @@ import type { CSSResultGroup } from 'lit';
 const toastStack = Object.assign(document.createElement('div'), { className: 'sl-toast-stack' });
 
 /**
+ * @title Alert
+ * @summary Alerts are used to display important messages inline or as toast notifications.
+ *
  * @since 2.0
  * @status stable
  *