Improve documentation

[felixhekhorn]

• provide more examples
• Mellin path argument choice N3LO matching #83 (comment) @giacomomagni
• reshuffle DGLAP
  • the page is too long and too complex
  • strip the j definitions (and refer to their docstring)
• reshuffle pQCD
  • the page is too long and too complex
  • e.g. split in splitting functions (including N3LO), couplings, masses, sv