Stop using Makefile and promote building with sphinx-build only

[astromatt]

Feature request / idea

• Makefile requires lots of additional dependencies
• This is particular a problem on Alpine Linux (docker container).
• Standalone sphinx-build works very well without any external dependency.
• Stop generating Makefile for new projects and move all sphinx-build arguments to conf.py.

Describe the solution you'd like

• Start promoting use of sphinx-build -c conf.py.
• If flag -c <FILE> is not specified, sphinx-build should look in current directory for conf file.
• All sphinxopts, sourcedir and builddir should be in conf.py.
• sphinxopts should be configuration variables not just simply "list of opts to pass to sphinx-build"
• This will also give opportunity to have multiple configuration in one project.

[tk0miya]

The same idea was posted to #5618. Could you post a comment to it please?