[Refactor] Refactor the doc structure (#1786)

* fix structure

* add version for m2r

* read docs

* add log

* api.rst

* fix api.rst

* fix the doc

requirements/docs.txt

 docutils==0.16.0
-m2r
+m2r==0.2.1
 mistune==0.8.4
 myst-parser
 -e git+https://github.com/open-mmlab/pytorch_sphinx_theme.git#egg=pytorch_sphinx_theme