Add GraalPy guides

[timfel]

• Deal with all "todo" left in the guides.

[steve-s]

@msimacek @fniephaus all todos should be resolved. We will update the "Next steps" sections with more links to in depth docs later when they are done. For now, this should be good to go. I've also added 3 new guides:

• native extensions guide (already reviewed in bb)
• freezing Python dependencies guide (already reviewed in bb)
• custom venv guide: (new) people repeatedly ask about this, and we also often tell people to try manually created venv, when installation of some package via Maven fails, so I think it would be handy to have a document that we can point people to

[fniephaus]

Thanks, @steve-s! I have moved all guides over to https://github.com/graalvm/graal-languages-demos/tree/main/graalpy.