Automatic doc generation for SofaPython3 plugin

[hugtalbot]

Today RST doc is in the /doc folder of the SofaPython3.
It needs to be manually compiled and uploaded on readthedoc to update the documentation.
This should be simplified.

It could also be the occasion to improve the doc.

[hugtalbot]

Feedback from @damienmarchal : [PoC working here](hugtalbot (Hugo): https://sofapython3.readthedocs.io/en/xp-readthedocs/)

• The fastest would be to have a ci process like the generate-doxygen which generate an artifact with the build documentation
• Then in readthedocs we just have to download that artifact and copy it to the right location (this what I did in the xp-readthedoc)
• The only missing part is make the ci script (which shouldn't be super hard for anyone that know how our jenkins ci is operating)

[hugtalbot]

Solved, since sofa-framework/SofaPython3#482 and sofa-framework/SofaPython3#484