feat: add 'aside' as a shortcode (#6)

The README has been updated to show a code example of both, including an
exemplary screenshot.

Since Hugo 0.55, we have to use `markdownify` in shortcodes that contain
HTML elements. At least we have to, if we want to render markdown code
inside the HTML element.

To use the `<aside>` element, we have to wrap our content in an
`<article>` tag.

This fixes #4.

Co-authored-by: Robert Lützner <robert.luetzner@iternity.com>

layouts/_default/single.html

@@ -2,5 +2,7 @@
 <h1>{{ .Title }}</h1>
 {{ partial "metadata.html" . }}
 <!-- <br><br> -->
+<article>
 {{ .Content }}
-{{ end }}
+</article>
+{{ end }}

layouts/index.html

@@ -1,5 +1,7 @@
 {{ define "main" }}
 {{ partial "metadata.html" . }}
 <!-- <br><br> -->
+<article>
 {{ .Content }}
-{{ end }}
+</article>
+{{ end }}

layouts/shortcodes/aside.html

@@ -0,0 +1 @@
+<aside>{{ .Inner | markdownify }}</aside>