docs: clean up and improve documentation

[shuuji3]

Description of the Change

Hello, I've read through all the documentation pages and refactored them to have consistent markups. Also, fixed several errors during the proofreading.

All changes are documentation clean up so I combined the same kind of changes into each commit, but if you feel this PR contains too many changes, I can split this PR into small chunks. 🙂

Checklist

• [ ] PR only contains one change (considered splitting up PR)
• [N/A] unit-test added
• [x documentation updated
• [N/A] CHANGELOG.md updated (only for user relevant changes)
• [ ] author name in AUTHORS

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 97.56%. Comparing base (817eb40) to head (90bddf9).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1401 +/- ## ======================================= Coverage 97.56% 97.56% ======================================= Files 32 32 Lines 2132 2132 ======================================= Hits 2080 2080 Misses 52 52 ```

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[shuuji3]

Thanks for merging!

Actually, switching single backtick to double ones is my initial motivation to make this PR. While they are almost the same in source code text, it is rendered as italic and code-block rspectively in the Read the Docs website (in HTML, <cite>italic</cite> and <code>code-block</code>).

I think the later presentation improves the readability much better (also <cite> is not appropriate as an HTML tag).

Before

https://django-oauth-toolkit.readthedocs.io/en/stable/rest-framework/getting_started.html

After

https://django-oauth-toolkit.readthedocs.io/en/latest/rest-framework/getting_started.html#step-1-minimal-setup