Apply Black and ISort, rearrange the documentation sources

[dmgav]

Changes:

• Apply black and isort to the code.
• Rearrange the documentation source files: the documentation is now in docs directory, which contains source and build directories. All source files are in docs\source directory. All the autogenerated source files are in docs\source\generated directory. The docs\build and docs\source\generated directories are excluded in .gitignore.

The PR contains no meaningful changes in the source code.