Add target parameter to figure shortcode

Add the "target" parameter to the figure.html shortcode.
If defined then add this target property to the href of the figure,
which allows the link to, e.g., open in a new tab.

Add description of target to docs

layouts/shortcodes/figure.html

@@ -7,6 +7,7 @@
   {{ $caption := .Get "caption" }}
   {{ $href := .Get "href" }}
   {{ $class := .Get "class" }}
+  {{ $target := .Get "target" }}
   {{ $nozoom := .Get "nozoom" | default false }}
   {{ if findRE "^https?" $url.Scheme }}
     <figure>
@@ -22,7 +23,7 @@
     {{ end }}
     {{ with $resource }}
       <figure {{ with $class }}class="{{ . }}"{{ end }}>
-        {{ with $href }}<a href="{{ . }}">{{ end }}
+        {{ with $href }}<a href="{{ . }}" {{ with $target }}target="{{ . }}"{{ end }}>{{ end }}
           {{ if $disableImageOptimization }}
           <img
             class="my-0 rounded-md{{ if $nozoom }} nozoom{{ end }}"