Support For Collections

[mmenanno]

Is your feature request related to a problem? Please describe the problem you're trying to solve.

Currently if you have pages in a collection, this gem does not apply the layout automatically

Describe the solution you'd like

Support for collections

Describe alternatives you've considered

Alternative is specifying the layout in the frontmatter

Additional context

I've put together an example of this using the just-the-docs template.
Repo: https://github.com/halorrr/collections-test
Site: https://halorrr.github.io/collections-test/

You can see when visiting the site everything looks as expected initially:
https://halorrr.github.io/collections-test/

If you click the Collection Test Layout page which has the layout specified in the frontmatter, it will also look normal:

But if you click the Collection Test No Layout page which does not have the layout specified in the frontmatter you will see not layout is applied:

This was a very basic test, setting up the collection the way that Just The Docs suggests:
https://just-the-docs.com/docs/configuration/#document-collections which you can see in the example config here:
https://github.com/halorrr/collections-test/blob/main/_config.yml

[mmenanno]

Created a quick PR for this, still need to figure out how the testing in the fixture setup works so I can add tests, but it seems to work when I tested locally #33

[mmenanno]

Tests on the PR to resolve this all seem to be working now, so it is ready when you have a chance to look @benbalter