Improve the chapter on working on the scipy-lectures

[GaelVaroquaux]

• The chapter is in the 'guide' directory
• It should explain quickly
  • How to build (using sphinx)
  • How to doctest
  • The use of examples to generate figures (ie: figure are not allowed to be checked in)
  • The use of intersphinx to link across projects
• It should stress our style guide:
  • Chapters should be readable / doable as a tutorial in 1.5 hours to 2 hours max (which implies limiting the amount of information)
  • Links (internal, to others projects and to Wikipedia) matter terribly
  • Jargon is to be avoided at all cost
  • The use of restructured text directives, such as topic, note, seealso, tip, sidebar, is strongly encouraged

[emmanuelle]

I did not see this issue before I started working on #260. Some points listed above have been fixed but not all of them.