Add TOC to installation instructions

This adds a Table of Contents to the installation instructions to
improve the navigation within the document.

I decided to use the command line to configure the TOC because there
were problems with using just using an attribute entry in the document
header.
Specifically the insertion of the `header.asciidoc` into the resulting
HTML file prevented the attribute entry approach from working.

The TOC can now be inserted into any doc file using

    toc::[]

INSTALL.asciidoc

@@ -1,6 +1,8 @@
 Installing qutebrowser
 ======================
 
+toc::[]
+
 On Debian / Ubuntu
 ------------------
 

scripts/asciidoc2html.py

@@ -184,7 +184,7 @@ class AsciiDoc:
         with open(modified_src, 'w+', encoding='utf-8') as final_version:
             final_version.write(title + "\n\n" + header + current_lines)
 
-        self.call(modified_src, dst, '--theme=qute')
+        self.call(modified_src, dst, '--theme=qute', '-a toc', '-a toc-placement=manual')
 
     def _build_website(self):
         """Prepare and build the website."""