API docs: LSIF: add references/definitions to tree API, not just blob API

Created by: slimsag

Prior to this commit I had defined documentationDefinitions and documentationReferences on just GitBlobLSIFData, but that requires you have a specific file path you're interested in. A more common case will be having a documentation path ID - and not having a file path at all - and wanting to know the references/definition locations from that.

I've implemented this here by defining it on TreeEntryLSIFData, alongside the other path-relative resolvers: documentationPage and documentationPathInfo.

Helps #21932 (closed)