[2.x] Clean up the navigation documentation

[emmadesilva]

Part of #1818 and goes over the entire navigation documentation to ensure it's consistent and concise, without losing information.

Additionally, it rethinks the document outline to account for many people not reading the document top to bottom, and instead only read sections at a time. As such, words that tie together sections have been removed, in order to make each section usable on its own.

It also updates headings to be more concise, instead of fluent English, to make the sections more skimmable.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (3f92d37) to head (b47bae0).

Additional details and impacted files

@@                     Coverage Diff                      @@
##             unify-the-navigation-api     #1828   +/-   ##
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  Coverage                      100.00%   100.00%           
  Complexity                       1855      1855           
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  Files                             190       190           
  Lines                            4909      4909           
============================================================
  Hits                             4909      4909           

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