Docs improvements

[rpkilby]

• [ ] Add tutorial
• [ ] Add topics
  • [ ] Explain how filters use standard Django form fields for input validation
• [ ] Add recipes
  • [ ] Add Form.clean example for together/exclusive handling
• [ ] Move current "reference" docs into an "API" section, as it's more conceptual than strict reference.
  • [ ] Reference Django's docs, (eg, the form docs)
• [ ] Add autodoc reference, this should become the actual "reference" docs

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older details, ref #402

• [x] Move everything except the first paragraph of "The Filter" section to the FilterSet Guide. This could be supplemented with a slightly longer introduction and maybe a TOC or subset of links to the FilterSet Guide? No longer necessary
• [x] (possibility) MethodFilter section in the usage docs is fairly lengthy, this could be cut down and moved to the filter reference.
• [ ] Document how to override filterset classmethods. ref django-filter iss 776

[rpkilby]

Evaluate prose/docs linting

• write-good
• proselint
• https://css-tricks.com/words-avoid-educational-writing/