Step 4: A PEP517-styled quickstart user guide and room for collaboration

[alvyjudy]

This PR should now have incorporated/complemented what's made in #1738 and the build_meta.txt file but I will check more thoroughly on that. Please let me know there were mistakes in this documentation too:) I have tried my best to go through the source code to see how keywords in setup.cfg are handled and tested out my own examples, but there might still be things I overlooked. If there are suggestions please let me know I'm more than happy to have other people's input.

Summary of changes

Step 4 on #2093
Proposed a quickstart user guide that adheres to PEP517 and provides a brief overview on the following functionalities of setuptools, with each section pointing to another page (WIP) that covers the details and more advanced usage.

1. installation
2. basic use
3. Automatic package discovery
4. Entry points and automatic script creation
5. Dependency management
6. Including Data Files
7. Uploading your package to PyPI
8. Transitioning from setup.py to setup.cfg
9. Resources on Python packaging

Since each section requires a more thorough documentation on the nuances and advance usage, this should make it easier for others to hop in and contribute. For example,

• document the backward-incompatible PEP 440 change to requirement semantics #310 can go into section 5
• Ignoring package_data when include_package_data is True #1461 can go into section 6
• Add mechanism for side-by-side comparison of setup.py and its equivalent setup.cfg #1699, Update setup.cfg documentation to mention the minimum version required for each setting to be supported #1535, Automatic conversion of setup.py to setup.cfg #1344, can go into section 8 (and for Moved distutils' doc for setup-keywords into our doc to document all supported keywords #1765 it should be linked to in this section but reside in a dedicated folder references)
• a new section could be created for Create usage instructions and rearrange docs #316

Pull Request Checklist

• Changes have tests
• News fragment added in changelog.d. See documentation for details