reference: remove "description" header, and use it for "extended description"

The "description" header included the short description of each command. In most
cases, this description was very short ("run a container"), and adding the extra
header before it only was adding extra noise.

This patch:

- removes the top "description" header
- renames the existing "extended description" header to "description" (a hidden
  "extended-description" anchor is added for backward compatibility)

As the extended description can be long (hopefully!), there may be a long distance
between the `Usage` section and the `Options` section. To help users navigate
to the list of available options, an extra line is printed if an extended
description is available for the command, including a link to the corresponding
section:

> Refer to the options section for an overview of available OPTIONS for this command.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

_includes/cli.md

@@ -2,7 +2,7 @@
 {% assign controller_data = site.data[include.datafolder][include.datafile] %}
 {% assign parentPath = page.path | prepend: "/" | remove: page.name %}
 
-## Description
+<br />
 
 {{ controller_data.short | replace_relative_links: page.path }}
 
@@ -77,8 +77,13 @@ $ {{ controller_data.usage | replace: tabChar, "" | strip }}{% if controller_dat
 
 {% endif %}
 {% unless controller_data.long == controller_data.short %}
+{% if controller_data.options %}
+Refer to the [options section](#options) for an overview of available [`OPTIONS`](#options) for this command.
+{% endif %}
 
-## Extended description
+## Description
+
+{: name="extended-description"}
 
 {{ controller_data.long | replace_relative_links: page.path }}