Styling improvements in descriptor tutorial
INTEGRATE: Documentation split into multiple files

* dev:
  Styling improvements in descriptor tutorial
  Descriptor tutorial improvement
  CONTRIBUTING file
  Contribution docs are in their own file
  API docs are in their own file
  Better placeholder documentation site
New API introduction
Frame and quixote module API docs in their own files
QElement API docs in its own file
Descriptor API docs are in their own file