add docs menu sections

[erquhart]

• Splits docs menu into "Quick Start", "Guides", "Reference", and "Contributing" sections.
• Uses Hugo's built in menu functionality
• Removes extraneous list.html template for docs and updates single.html to cover all cases (we don't actually have a list view, it was mostly duplicate code).

This will need to be ported into the upcoming Gatsby rewrite (cc @zebapy), but this is from a while ago and should be replicated in any migration.

[verythorough]

Deploy preview for cms-demo ready!

Built with commit e684896

https://deploy-preview-1366--cms-demo.netlify.com

[verythorough]

Deploy preview for netlify-cms-www ready!

Built with commit 0bed27e

https://deploy-preview-1366--netlify-cms-www.netlify.com

[verythorough]

Deploy preview for netlify-cms-www ready!

Built with commit e684896

https://deploy-preview-1366--netlify-cms-www.netlify.com

[verythorough]

Woot! Glad to see this improvement. I may propose some changes to the groupings, but any grouping at all is a huge boost. :)

[zebapy]

@erquhart So architecture.md and customization.md are excluded from menu on purpose now?

[verythorough]

Ah, good point! I suspect this may be because these changes were originally part of the Preview API docs PR, #1311. Until the new Preview API is implemented, though, I imagine we want to keep the docs for the current API in place.
I'm thinking maybe Architecture could go under "Contributing," and customization.md would be "Creating Custom Previews" under "Guides."

[erquhart]

Good catch @zebapy, I'll add those back.