Add Nav documentation and introduce NavLayout enum for layout options.

[terabytesoftw]

Q	A
Is bugfix?	✔️
New feature?	❌
Breaks BC?	❌

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.65%. Comparing base (dda8d28) to head (1dab0ba).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #278   +/-   ##
=========================================
  Coverage     99.65%   99.65%           
  Complexity      668      668           
=========================================
  Files            23       23           
  Lines          2863     2863           
=========================================
  Hits           2853     2853           
  Misses           10       10           

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