Support hierarchical index entries

[Phrogz]

The index should be able to have entries like:

Rigid Bodies
   Basics
   Creating
   Rigid Body Modifier

Presumably this would only be through a meta entry—using some TBD syntax—and not through the auto scanner.

[Phrogz]

Perhaps a slash in the keyword entry.

[Phrogz]

Like: keywords: Rigid Bodies/Basics

[Phrogz]

It would be really nice to be able to have index entries target sections on the page; hack that into the existing comma-delimited single keywords entry, or have more explicit syntax, perhaps one line per keyword?

[Phrogz]

All the work to add the LinkTree, and I forgot this is why I did it. The Index still needs to be converted to a LinkTree.