Implement the compilation of the docs and the notebooks

[jlperla]

• [x] Remember to cleanup the documenter .jl file for building docs to make it more standard.
  • Check environment variable that travis might set
  • Make sure that runs on a local machine for testing (e.g. if the tag enivornment variabler isn't set, it doesn't break).
• [x] Do you know which tag your on when running documenter to generate a link?
• [x] Change the links on the README to point at the right things. i.e. on the front page is https://quantecon.github.io/SimpleDifferentialOperators.jl/stable/generated/discretized-differential-operator-derivation.pdf Change this to something else, make it the "dev" or "master" one
• [x] Change the docs themselves to point at the releases for the notebooks and the pdf
  • Work for both the master and the tagged versions if possible.
• [x] After the whole thing is setup, we need to change the links to the latest docs in the PTW repository/tex file.

[arnavs]

Did what's required here, and updated the links from PTW.