Miscellaneous documentation improvements

[moggers87]

Description

Fixed a few minor mistakes in documentation and did a little reformatting to make things neater. Also added a "docs" test environment to our tox configuration so developers don't have to manually install sphinx to build documentation.

Motivation and Context

While answering #761 I noticed a mistake in the documentation and while fixing that I noticed another mistake etc. etc. and now I have a handful of commits 😸

How Has This Been Tested?

Ran make docs locally and skim read the documentation.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.03%. Comparing base (8f02f48) to head (c0d4351).
Report is 10 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #762      +/-   ##
==========================================
+ Coverage   97.95%   98.03%   +0.08%     
==========================================
  Files          76       76              
  Lines        3516     3510       -6     
  Branches      177      177              
==========================================
- Hits         3444     3441       -3     
+ Misses         43       40       -3     
  Partials       29       29              

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[moggers87]

The readthedocs config should work, but we won't know for sure until this PR is merged. Looking at the long list of failed builds I don't think we'd be losing much if it doesn't work for whatever reason: https://app.readthedocs.org/projects/django-two-factor-auth/builds/

[moggers87]

Would be nice to enable RTD pull request builds IMO.

[moggers87]

This isn't anything to do with documentation but it was right there looking at me so I fixed it

[claudep]

Wow, nice work! 👍

[claudep]

And RTD build fixed, kudos!