Improve image transcript navigation

- Identify the transcript with a subheading, ending in a period so that
  the aria-description separates the identifier from the concept.
- Put the transcript in a <section>
- Snuck in a trivial syntax fix for the generator element.

layouts/partials/head.html

@@ -63,5 +63,5 @@
 <meta property="og:image:alt" content="White-on-black colon and semicolon">
 <meta property="og:url" content="{{ .Site.Params.CanonicalBaseURL }}{{ $canonicalRelPermalink }}">
 <meta property="og:description" content="{{ $description }}">
-{{ hugo.Generator }}
+{{ hugo.Generator | replaceRE " />" ">" | safeHTML}}
 </head>

layouts/shortcodes/transcribed-image-transcript.html

@@ -1,9 +1,11 @@
 <details>
 <summary>Toggle transcript</summary>
-<div itemprop="embeddedTextCaption"
 {{ with .Parent -}}
-	id="transcript-{{ .Get "id" }}"
-{{- end -}}>
+<section id="transcript-{{ .Get "id" }}">
+<h5>Image transcript.</h5>
+{{- end -}}
+<div itemprop="embeddedTextCaption">
 {{ .Inner | markdownify | safeHTML }}
 </div>
+</section>
 </details>