Sphinx and Doxygen documentation

[diptorupd]

Adds an initial infrastructure to generate API documentation. Documentation generation works fine locally. Tried publishing them on gh-pages with not so great results.

I think Sphix+breathe+exhale+doxygen works much nicer than raw doxygen. Although, some information that is on doxygen is not showing up in sphinx using exhale and breathe.

Checkout xtensor documentation to see a more polished example than what I have managed to do thus far using breathe and exhale.

[diptorupd]

Pushed the static pages to gh-pages. You can see them rendered here https://intelpython.github.io/dpctl/. Somehow they render incorrectly.

[oleksandr-pavlyk]

Trying to compile with build_for_develop, I encountered:

CMake Error at CMakeLists.txt:192 (add_subdirectory):
  add_subdirectory given source "docs" which is not an existing directory.


-- Configuring incomplete, errors occurred!

[PokhodenkoSA]

Pushed the static pages to gh-pages. You can see them rendered here https://intelpython.github.io/dpctl/. Somehow they render incorrectly.

I would like to propose URL like for SDC and add -doc suffix: https://intelpython.github.io/dpctl-doc/

@Alexander-Makaryev what are URLs for dpNP and daal4py?

[PokhodenkoSA]

Resolved merge conflict:

1. Formatted with black - master is black formatted
2. Merged with master

[PokhodenkoSA]

I have fixed incorrect rendering. Just created .nojekyll file. See this commit 99e7f9b and its commit message.

[diptorupd]

@oleksandr-pavlyk Can you review the PR once more? I have fixed the CMakeLists.txt issue you found. I will like to merge this as the first step to making generating docs easier.

[diptorupd]

@oleksandr-pavlyk Actually let me move the changes to backends\CMakeLists.txt and sycl_core.pyx out of the PR.

[diptorupd]

@oleksandr-pavlyk Can you please review the changes I made to the Cython files to add __all__? I will like to merge this PR as it makes generating docs much easier.

[oleksandr-pavlyk]

LGTM.