feat(alert): simplify usage with Markdown content

[razonyang]

Previously, we have to wrap the Markdown content with markdownify shortcode as follows.

{{< bs/alert primary >}}
{{% markdownify %}}
A simple *primary alert* with Markdown—**check it out**!
{{% /markdownify %}}
{{< /bs/alert >}}

Now, with this PR, we can simplify the syntax.

{{% bs/alert style=primary %}}
A simple *primary alert* with Markdown—**check it out**!
{{% /bs/alert %}}

Replace < symbol with %.

But this requires you import a minor style to remove the margin-bottom of last <p> element.

.alert-content {
  > p:last-child {
    margin-bottom: 0 !important;
  }
}

You can either import the mods/bootstrap/scss/index SCSS via Hugo Pipes.

As to HB Framework, you just need to upgrade github.com/hbstack/bootstrap.