Add topic guides based on Discourse Discussion

Topic Guides

[ ] Packaging a pure-Python project with basic defaults Walk through the simplest possible case. Given some of the more important definitions. This needs to mention that backends, etc exists, and link to other guides. It is important to convey to beginners that if they hit a wall with simplest things, that there is more to look into, and some idea where.
[ ] Using and choosing build backends Has many definitions: what is a backend, why does it matter? Discussions and comparisons of common/popular backends and their configuration, at least one full example, and lots of links to backend projects docs.
[ ] Handling advanced cases with compilation steps A couple of full examples, discussion and links of tools like scikit-build or advanced build capabilities of other backends. There’s lots to potentially cover here, or at least mention and link to things about: cython, wrapping C, C++ or Fortran, building/bundling Typescript. Maybe suggest DPO as a place for deep technical questions.
[ ] Publishing packages Information about PyPI and twine.
[x] Modernizing legacy setup.py usage Address all the things that came up for the OP here, links to the other guides as appropriate. Quick tips/FAQs for common things e.g. “what do I do with these command line arguments?”

pypa / packaging.python.org