Add overview/guide to the docs

[florence]

Facilities like dc and 2htdp/image being pict-convertible? are currently hard to discover.

@rfindler suggested that it seems like a good idea to add in a guide like overview of pict, which i imagine mentioning:

Combinator like image drawing
Baselines, text, and pict-last and how to use them
pict/convert and its implementations in the main distro
Using pict with scribble

Ill try to add this soon, and would love suggestion as to what else should go in this guide!

[bennn]

• How to read & write image files (png, jpg)
• A small example of pin-line or pin-arrow-line.

Here's a little shopping bag that uses pin-line:

#lang racket

(require pict)

(define w 100)
(define h 160)
(define line-width 5)
(define line-color "brown")
(define turn (* 2 pi))

(define bag
  (filled-rectangle (* w 3/4) (* h 1/2) #:color "chocolate" #:border-color line-color #:border-width line-width))

(define handle-anchor
  (blank (* w 1/2) 0))

(pin-line
  (cc-superimpose
    (filled-rectangle w h #:color "white" #:draw-border? #f)
    (vc-append handle-anchor bag))
  handle-anchor lt-find
  handle-anchor rt-find
  #:start-angle (* 24/100 turn)
  #:end-angle (* 74/100 turn)
  #:start-pull 1/2
  #:end-pull 1/2
  #:line-width line-width
  #:color line-color)