Improve Oscar dev docs

[fingolfin]

• rename some sections to drop the "OSCAR" reference, e.g. "Debugging OSCAR code" -> "Debugging code" or "Hints for debugging" or so
• merge the two "printing" sections, namely https://docs.oscar-system.org/dev/DeveloperDocumentation/printing_details/ and https://docs.oscar-system.org/dev/DeveloperDocumentation/styleguide/#Printing-in-Oscar (I would keep the one on its own page and merge into it the one which is just a subsection of a longer pagef)
• document the two Oscar.doctest_fixup methods (at least pull in their doc strings), perhaps in https://docs.oscar-system.org/dev/DeveloperDocumentation/documentation/
• ...

[fingolfin]

@emikelsons maybe you can also look into this?

[emikelsons]

Yes, I can look at this together with #4657