Docs: Add guide for non-admin polymorphic views

[JohananOppongAmoateng]

This PR addresses #346 by adding documentation on how to handle polymorphic forms in standard Django views (outside the admin).

Closes #346

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.98%. Comparing base (06729c3) to head (0ff5a43).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

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  Coverage   83.98%   83.98%           
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  Files          27       27           
  Lines        1611     1611           
  Branches      253      253           
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  Hits         1353     1353           
  Misses        196      196           
  Partials       62       62           

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[bckohan]

Thanks for the solid start on this @JohananOppongAmoateng

I made two major changes that you should take note of because they'll apply to other PRs:

1. I put the example code into a test app (tests/examples/views) and then referenced it in the docs using literalinclude. The code is tested in the test.py file in that app. I'd like to follow this pattern for all code in the docs so we can be assured everything is tested.
2. I rewrote the example to key off of model label instead of content type id. For version 5 we'll have to make this change for all the forms and so will users who've written their own forms.