Feature Request: Allow for types index page to be the 'homepage'

[mphuff]

See Issue #11.

If the types page gets added, it would be nice to have an option when generating the avrodoc file to specify that the types index page should be set to the homepage rather than the protocols index page.

We have a number of generated avrodoc files that have a couple of empty protocol definitions that simply contain slews of type definitions. These are the docs we are shipping to customers and it would be nice to have them displayed at the homepage rather than having to click through to each type to get a glance of what is available to them.