Add a quickstart to the development chapter of the user guide

It covers using cabal init and the basic steps for building from there.
Rewrite and extend several parts of the user guide introduction

Introduce cabal-install much earlier. Give more of a basic intro.
Try using a contents within each user guide chapter

Currently we have a somewhat hard-to-maintain contents just in the
introduction. This cuts that initial contents down to just link to
the chapters (plus sections within the intro), and then use an
automatically generated table of contents in the other chapters.

Also tweak the CSS because pandoc's TOC feature turns all section
headings into links and that changes the visual appearance. Use the
CSS to make it look the same as before.
Limit the length of lines in the user guide html to be readable
Add a user guide section on package concepts