Make navigation behaviour for nested pages configurable

[caendesilva]

Implements the data generation backend for this feature. We are still going to need to actually implement frontend code to support automatic dropdowns.

#669

Say we have a structure like this, where pages are in a directory of their source directory:

_pages/about/foo.md
_pages/about/bar.md

How should Hyde handle the navigation for these pages? Currently they get added to the navigation menu, but maybe they shouldn't? I have a few ideas for improvement, and I think this should be configurable. I am very happy to get input on this, and also which option should be the default.

1. Add the pages to the navigation menu (current behaviour)
2. Put them in an automatic dropdown based on the subdirectory name (my fav)
3. Don't show them at all

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@1a998ae). Click here to learn what that means.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             master      #740   +/-   ##
==========================================
  Coverage          ?   100.00%           
  Complexity        ?      1134           
==========================================
  Files             ?       137           
  Lines             ?      2771           
  Branches          ?         0           
==========================================
  Hits              ?      2771           
  Misses            ?         0           
  Partials          ?         0           

Impacted Files	Coverage Δ
.../src/Framework/Factories/NavigationDataFactory.php	100.00% <100.00%> (ø)
...c/Framework/Features/Navigation/NavigationMenu.php	100.00% <100.00%> (ø)

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[caendesilva]

We are still going to need to actually implement frontend code to support automatic dropdowns