Eliu gh 353 make autodoc pipeline recognize new gpu docs

********* PEOPLE *************
Primary reviewer: @sc932 

Reviewers: @garywang304 

********* DESCRIPTION **************
Branch Name: eliu_gh_353_make_autodoc_pipeline_recognize_new_gpu_docs
Ticket(s)/Issue(s): Closes #353

    1. define version string
    2. make autodoc tools understand the gpu directory
    3. cleanup some doc warnings & rendering issues
    4. updating contributing.rst to include version bump steps

@sc932 major files to see are any .md and .rst files, conf.py, doxygen_config, and cpp_rst_maker.py. 

hpp/cpp changes are all doc changes.

@garywang304 please check over changes to the ```*gpu*``` files and ```*cuda*``` files.

********* TESTING DONE *************
make test
make -B docs (visual checks)
you should do the MAJOR, MINOR, PATCH format thing and link to semver.org