Improve navigation priority discovery to allow sidebars and main navigation to support both list and keyed config syntax

[emmadesilva]

Previously, documentation pages supported the following for setting navigation priorities in the config:

'foo', // Gets priority 500
'bar', // Gets priority 501
'baz', // Gets priority 502

And the main navigation menu supported the following syntax:

'foo' => 10,
'bar' => 50,
'baz' => 75,

This PR makes it so both menu types support both config syntaxes, and even a combination of both:

'foo',        // Gets priority 500
'bar' => 100, // Gets priority 100
'baz',        // Gets priority 501

[codecov]

Codecov Report

Merging #1431 (ad97ba7) into master (96b1232) will not change coverage.
The diff coverage is 100.00%.

@@              Coverage Diff              @@
##              master     #1431     +/-   ##
=============================================
  Coverage     100.00%   100.00%             
- Complexity      1707      3420   +1713     
=============================================
  Files            180       360    +180     
  Lines           4633      9280   +4647     
=============================================
+ Hits            4633      9280   +4647     

Files	Coverage Δ
.../src/Framework/Factories/NavigationDataFactory.php	100.00% <100.00%> (ø)

... and 180 files with indirect coverage changes