deprecated_args decorator for sphinx formatting

[mjhajharia]

this PR is for doing a draft version of adding auto-generate docs for deprecated arguments.

[mjhajharia]

[ Things to note: from Oriol ] -> not sure that would work. There could be colons within the parameter description so option 1 would need extra work regarding indentation or something like this (example https://arviz-devs.github.io/arviz/api/generated/arviz.loo.html scale parameter). It also has to accept both name: type or name : type (which is actually the numpydoc compliant formatting) because we have both in the codebase. option two should work but needs a couple caveats: section names should be preceded by an empty line and the number of underlines should be the same as the characters of the section, so the ------- should take something like between 4 or 15 characters, not completely sure they need to be dashes but I do think we always use dashes. The main problem is option 1 which might become too complicated to handle by ourselves with regex or other simple things

[mjhajharia]

closing in favour of laurent-laporte-pro/deprecated#51