Update .gitignore to ignore the generated folder
Update .gitignore to ignore the generated folder

This folder is created when the documentation is built and pollutes the
output of git status.

jdrudolph/scikit-bio,xguse/scikit-bio,Kleptobismol/scikit-bio,jdrudolph/scikit-bio,SamStudio8/scikit-bio,Achuth17/scikit-bio,colinbrislawn/scikit-bio,Achuth17/scikit-bio,jensreeder/scikit-bio,wdwvt1/scikit-bio,xguse/scikit-bio,johnchase/scikit-bio,gregcaporaso/scikit-bio,Jorge-C/bipy,colinbrislawn/scikit-bio,anderspitman/scikit-bio,kdmurray91/scikit-bio,demis001/scikit-bio,demis001/scikit-bio,jairideout/scikit-bio,Kleptobismol/scikit-bio,averagehat/scikit-bio,SamStudio8/scikit-bio,jairideout/scikit-bio,johnchase/scikit-bio,corburn/scikit-bio,kdmurray91/scikit-bio,jensreeder/scikit-bio,wdwvt1/scikit-bio,corburn/scikit-bio,gregcaporaso/scikit-bio,Kleptobismol/scikit-bio,anderspitman/scikit-bio,averagehat/scikit-bio