[Docs] Update REAME and setup Sphinx (open-mmlab#2)

* add video in README

* add CITATION.cff

* add mmhuman3d link

* update video link

* add badge

* modify title size

* add sphinx generated docs

* add logo img in sphinx doc

* Update video link in README

* fix api.rst

* fix sphinx build documentation

* add additional license in index.rst

* replace source code links in docs with github url

* rm sphinx warn in cameras

* update docstring

* use github url in docs

* revise acc to mmpose conf.py

Co-authored-by: yanglei <[email protected]>
Co-authored-by: Maoxie <[email protected]>