Relax index file requirement in toc

[fwkoch]

Previous _toc.yml tocs required an index file to be valid. Similarly, our in-memory project structure requires an index file. This requirement makes some sense for site builds but does not make sense for exports (Currently, the index file is the default source of single-page exports but it is ignored for multi-page exports - very strange).

Since the new toc structure does not require an index file, we need to allow exports to define their toc with no index file, if that's the behaviour the user wants.

Further, projects need a way to infer an index file from a pattern top-level toc entry or maybe even a folder top-level entry. A few options:

• Just take the first page as the index. This doesn't really work for top-level folders, as it pulls the index out of the folder.
• Create a default index page
• Redirect from the / root of the site to the first page, which may be in a folder
• Or, we can just keep the index page requirement for projects/sites while relaxing it for exports - This is probably the easiest first pass.

Work on this issue will likely be directly tied in to #1243