build(docs-infra): add a link to `usage notes` from `description` (#40835)

The "Usage Notes" section is often at the bottom of a long page of content
which can make it difficult to discover. This commit adds a link at the
end of the "Description" if there is a "Usage Notes" section on the page.
This link should help the reader to discover that there is more information
available further down the page.

This is basically a marginal fix. The longer term fix should be to
consider what content goes in which sections and how they should
be laid out on the page.

See #40753

PR Close #40835

aio/tools/transforms/templates/api/includes/description.html

@@ -2,5 +2,7 @@
 <section class="description">
   <h2>Description</h2>
   {$ doc.description | trimBlankLines | marked $}
+  {% if doc.usageNotes %}<p>Further information available in the <a href="#usage-notes">Usage Notes</a>...</p>{%
+  endif %}
 </section>
 {% endif %}