Deploy Sphinx documentation to Github pages

[hexoul]

Motivation

• 33

• Unless users build Sphinx at local, there was no way to see documentation.

Modifications

• Add a Github action to deploy to Github pages.
  • It tested at forked repo. https://hexoul.github.io/centraldogma-python/centraldogma.html#module-centraldogma.dogma
• misc.
  • Fix pre-build command for Sphinx.
  • Fix wrong spaces.

Result

• Deployed Github pages.
• Close #33