Fixed michocott pdf generation

There was an issue where using make michelson.pdf in the *src/michocott" folder didn't generate the correct pdf for the documentation: it was generated using coqdoc, while we want to use the tex export features of ott

I found a workaround for this issue by adding a new target doc in the local Makefile. Using make doc in the src/michocott folder therefore generates the correct doc (provided the user has the right tools installed)

I also added a few missing stuff in the .gitignore