A folder's meta.json title is ignored in the sidebar #41

Summary

A folder's own meta.json can set title, but the sidebar shows a title made from the folder name instead.

Steps to reproduce

content/docs/maui/meta.json
{ "title": "MAUI (preview)", "pages": ["getting-started"] }

Expected: the sidebar section reads "MAUI (preview)".
Actual: it reads "Maui".

Where

src/ShellDocs.Core/NavigationGraphBuilder.cs builds folder sections with TitleFromFolderName(name); MetaJson.Title is parsed but not used for the folder itself. (Inline sub-sections declared in a parent's pages do use their title.)

Suggested fix

When a folder has a meta.json with a non-empty title, use it for that folder's section node. Add a NavigationGraphBuilderTests case.