doc: Page intro improvements

The introductionary sections of as-module and cli-options have been
rewritten. Further, Instaloader's short-description is now included in
the sidebar.

docs/_templates/caption.html

@@ -1 +1,2 @@
 <h2><a href="index.html">Instaloader</a></h2>
+<p>Download pictures (or videos) along with their captions and other metadata from Instagram.</p>