v2: Introduce show_overlay_excerpt and document (#1436)

Make excerpts optional in header overlay

Some posts and some header images don't lend themselves well to
displaying the excerpt in the overlay. Make this optional by
introducting a new boolean variable:

`page.header.show_overlay_excerpt`

Set it to implicitly default to `true` so existing users are unaffected by
testing explicitly for "!= false".

This can be enabled globally for a site by changing the default to `false`
in the local _config.yml, or per page by specifying the value in front
matter.

Document show_overlay_excerpt variable

* Add show_overlay_excerpt variable to the Header Overlay field documentation table.

Close #1429

_includes/page__hero.html

@@ -42,7 +42,7 @@
           {{ page.title | default: site.title | markdownify | remove: "<p>" | remove: "</p>" }}
         {% endif %}
       </h1>
-      {% if page.excerpt %}
+      {% if page.header.show_overlay_excerpt != false and page.excerpt %}
         <p class="page__lead">{{ page.excerpt | markdownify | remove: "<p>" | remove: "</p>" }}</p>
       {% endif %}
       {% if site.read_time and page.read_time %}