Clarify the introduction
Clarify the introduction

In particular:

  - Replace references to the 'PPrint' library to 'this library' or
    similar.  'PPrint' which was perhaps Daan Leijen's name for the
    original library, but it doesn't show up these days, and could
    merely confuse users of this library.

  - Hyperlink to identifiers mentioned in the introduction.

  - Clarify what "ribbon width" is.  (I had to read Wadler's paper
    to understand this; the original documentation left me confused.)

  - Remove the (now dead?) link to Daan's original library
    documentation: most of the content there was /already/ present
    in this module's documentation, and I've added what else was
    missing (specifically the document algebra).

  - Move the ANSI console formatting notes into the 'ANSI formatting
    combinators' section.

quchen/prettyprinter,quchen/prettyprinter