add auto gen. docs in gh-pages

[lukaszstolarczuk]

add automatic generation of gh-pages docs, using sphinx

and additionally, link to pmem.io

[lukaszstolarczuk]

ref. #42 & #44

[lukaszstolarczuk]

now, we switched to holding content of the documentation on pmem.io/pmemkv-python (it's done e.g. on pmemstream; pls see there to compare)