[2.x] Improved navigation group label generation

[caendesilva]

This targets v2.x via #1568 and picks up the idea behind #1575

This PR makes improvements to how navigation group labels are generated

This PR makes improvements to how navigation item labels are generated. The biggest change which could be breaking is that all labels are now formatted. So if you create a navigation item with the label "foo" it will now be normalized to "Foo". This may be opinionated, but considering that we do the same for page titles, why should this be different?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (d3a0a59) 100.00% compared to head (8cc5bd6) 100.00%.

Additional details and impacted files

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##             improved-navigation-internals     #1576   +/-   ##
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  Coverage                           100.00%   100.00%           
- Complexity                            1762      1763    +1     
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  Files                                  184       184           
  Lines                                 4776      4772    -4     
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- Hits                                  4776      4772    -4     

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[what-the-diff]

PR Summary

• Menu Item Grouping Implemented in Sidebar
  Updates have been made to the 'generate' method in GeneratesDocumentationSidebarMenu.php to better manage how navigation items are grouped together. This allows for a more organized display of items in the sidebar menu.

• Normalized Labels for Navigation Items
  Changes to NavItem.php include methods to normalize labels, find ideal group labels, and return group labels for items with subitems. This ensures consistency and clarity in how navigation items are presented.

• Removal of Redundant Method
  The normalizeGroupLabel method has been removed, as normalizeLabel now handles all necessary label formatting.

• Improved and Expanded Test Cases
  Test cases in both AutomaticNavigationConfigurationsTest.php and NavItemTest.php have been updated for navigation items in different contexts to ensure accuracy of labels and their grouping. New tests have also been added to check sidebar label formatting and label-setting in the configuration. This ensures our system handles various scenarios correctly.

[caendesilva]

It may violate the principle of least astonishment to always format the label. If the user manually creates a NavItem as new NavItem(label: 'foo'), the label should return foo. We only really want to change titles for autodiscovered items. And since the main navigation menu does this automatically for NavItems corresponding to pages, we should only do this for the sidebar group labels as it's just there we don't have a direct way of editing the front matter.

Edit: I think consistency wins over here.