Sphinx generated documentation (#521)

* upgrade req.

* move MkDocs

* create Sphinx

* init Sphinx

* move md from MkDocs to Sphinx

* CI: build docs

* build Sphinx

formatting

move docs from MD to docstring in particular package/modules

formatting

add Sphinx ext.

rename root_module to core

drop implicit name "_logger"

drop duplicate name "overwrite"

fix imports

use pytorch theme

add sample link mapping

try fix RTD build

use forked template

fix some docs warnings

fix paths

add deprecation warnings

fix flake8

fix paths

revert refactor

revert MLFlowLogger

* revert example import

* update link

* Update lightning_module_template.py