Add breadcrumbs for other manuals

- remove breadcrumbs from immigration
- remove a duplicated manual file
- use the manual in the document presenter to construct breadcrumbs
- extract section groups and sections to their own presenters
- adding a `manual_index.html.erb` to display just the Manual, instead of faking it as a Document
- adding slugs to placeholder manuals
- root breadcrumb is always manual.url but outside `document.breadcrumbs`, so that method will return `[]` unless `document['details']['breadcrumbs']` is present, which should only happen with HMRC manuals