Add alert blocks in markdown (#29121)

- Follows https://github.com/go-gitea/gitea/pull/21711
- Closes https://github.com/go-gitea/gitea/issues/28316

Implement GitHub's alert blocks markdown feature

Docs:
-
https://docs.github.com/en/get-started/writing-on-github/getting-started-with-writing-and-formatting-on-github/basic-writing-and-formatting-syntax#alerts
- https://github.com/orgs/community/discussions/16925

### Before

![image](14f7b02a-5de5-4fd0-8437-a055dadb31f2)

### After

![image](ed06a869-e545-42f1-bf25-4ba20b1be196)

## ⚠️ BREAKING ⚠️

The old syntax no longer works

How to migrate:

If you used
```md
> **Note** My note
```

Switch to
```md
> [!NOTE]
> My note
```

---------

Signed-off-by: Yarden Shoham <git@yardenshoham.com>
Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Giteabot <teabot@gitea.io>

web_src/css/base.css

@@ -1268,20 +1268,45 @@ img.ui.avatar,
   border-radius: var(--border-radius);
 }
 
-.attention-icon {
-  vertical-align: text-top;
+.attention {
+  color: var(--color-text) !important;
 }
 
-.attention-note {
-  font-weight: unset;
-  color: var(--color-info-text);
+blockquote.attention-note {
+  border-left-color: var(--color-blue-dark-1);
+}
+strong.attention-note, span.attention-note {
+  color: var(--color-blue-dark-1);
 }
 
-.attention-warning {
-  font-weight: unset;
+blockquote.attention-tip {
+  border-left-color: var(--color-success-text);
+}
+strong.attention-tip, span.attention-tip {
+  color: var(--color-success-text);
+}
+
+blockquote.attention-important {
+  border-left-color: var(--color-violet-dark-1);
+}
+strong.attention-important, span.attention-important {
+  color: var(--color-violet-dark-1);
+}
+
+blockquote.attention-warning {
+  border-left-color: var(--color-warning-text);
+}
+strong.attention-warning, span.attention-warning {
   color: var(--color-warning-text);
 }
 
+blockquote.attention-caution {
+  border-left-color: var(--color-red-dark-1);
+}
+strong.attention-caution, span.attention-caution {
+  color: var(--color-red-dark-1);
+}
+
 .center:not(.popup) {
   text-align: center;
 }