[SR-15947] Swift-DocC doesn't use render node navigator title when generating index

[franklinsch]

Previous ID	SR-15947
Radar	None
Original Reporter	@franklinsch
Type	Improvement

Additional Detail from JIRA

Votes	0
Component/s	Swift-DocC
Labels	Improvement
Assignee	None
Priority	Medium

md5: c4d2b97a7a3ab82dadcb561a4b36861b

Issue Description:

When generating a navigator index, DocC seems to be using the render node's `metadata.fragments` value instead of `metadata.navigatorTitle` for navigator item titles for Swift symbols. It's unclear why that's the case, so we should check if there is a valid reason for this, and if not, use `metadata.navigatorTitle`.

https://github.com/apple/swift-docc/blob/main/Sources/SwiftDocC/Indexing/Navigator/NavigatorIndex+Ext.swift#L83