Disable default layouts for documents with a layout: none declaration

[ashmaroli]

Inspired by discussion in #5929

While layout: null will still continue to be overridden by the defaults, with this pull in place, the user will be able to process and output a Jekyll::Document object without being placed into a layout by
simply declaring layout: none in the Front Matter

/cc @jekyll/ecosystem

Let me know if this results in any unintended behavior.. I'll update as required.

[parkr]

This should be layout: null. I don't know if I like us fiddling with the way YAML works.

[ashmaroli]

This should be layout: null. I don't know if I like us fiddling with the way YAML works.

IMO, the proposed approach is better because it introduces a new route and only affects Jekyll::Document objects..
Modifying layout: null will require modification of #deep_merge_hashes which is used by various other objects

[parkr]

Ugh, I don't love this. If I had a _layouts/none.html file and layout: none today, then after this PR, I wouldn't have a layout on my page anymore. That seems like a breaking change.

A brief search shows that it's probably safe....

@benbalter, I think this could be an acceptable breaking change. Thoughts?

[mattr-]

IMHO, this is definitely a breaking change. I'm also not sure what the use case for this would be when we have the standard YAML version already in layout: null.

[pathawks]

we have the standard YAML version already in layout: null.

That's the problem: we don't. When we merge the defaults into a document’s front matter, we will override layout: null the same way we would if no layout were specified at all.

[parkr]

@jekyllbot: merge +minor