Use index.md for subfolder parents in documentation to get nicer links

[m3nu]

Is your feature request related to a problem? Please describe.
Current documentation recommends putting children in a common folder and naming the parent residing in the same folder e.g. utilities/utilities.md.

This results in fairly weird permalinks, e.g. example.com/utilities/utilities.

Describe the solution you'd like
I tried using index.md for the parent page in a sub folder and it works as well AND gives nice links. E.g. example.com/utilities. To use the example it would look like this:

|   |-- utilities
|   |   |-- index.md      (parent page, now available under /utilities or /utilities/index.html)
|   |   |-- color.md
|   |   |-- layout.md
|   |   |-- responsive-modifiers.md
|   |   +-- typography.md

If there are no technical reasons against naming the parent page index.md, I would go ahead and make a PR for this to update the documentation. It seems no code changes are needed as it already works when used.

[pmarsceill]

Sure, go for it 👍

[m3nu]

Great. Added a suggested PR that updates the documentation at #181.