Update documentation about packaging theme to use modern tools

[sumau]

The documentation about packaging theme refers to somewhat outdated tools and practices.

I was wondering if you could update the documentation to use pyproject.toml and perhaps setup.cfg:
https://packaging.python.org/en/latest/tutorials/packaging-projects/

I have an example pyproject.toml file:

[build-system]
requires = ["setuptools"]
build-backend = "setuptools.build_meta"

And example setup.cfg:

[metadata]
name = mkdocs-<theme-name>
version = attr: <theme_name>.__version__
author = 
author_email = 
url = 
description =  
long_description = file: README.md
long_description_content_type = text/markdown
keywords = mkdocs
license = MIT
classifiers =
    Programming Language :: Python :: 3
    License :: OSI Approved :: MIT License
    Operating System :: OS Independent

[options]
packages = find:
include_package_data = True
zip_safe=False
install_requires =
	mkdocs-<theme-parent>

[options.entry_points]
mkdocs.themes =
	<theme_name> = <theme_name>

[options.extras_require]
docs =
	markdown-include
	mkdocs

[options.package_data]
<theme_name> =
    *.html
     mkdocs_theme.yml

[ultrabug]

Hi @sumau ; it would be useful if you could propose that change through a PR maybe?