Reinstate table of contents

Attempting to build the HTML files resulted in the following error:

```
$ perl build/tools/build_html.pl 
Cannot open file [build/chapters/chapter_toc.pod]: No such file or directory
```

Going through the build_html.pl script and uncommenting all the references to the table of contents (usually 'TOC') and then re-running script removed the error and built a working set of HTML documents.