feat: optional catch all routes

[frouo]

Context

At the time of writing, next-markdown support nextjs "catch all routes", meaning you need to add a [...nextmd].js in your pages and create an index.js for the home page.

Purpose

We want to create a website (including the home page!) with markdown. To allow creating home page we need to use "optional catch all routes".

Todo

next-markdown should work with [[...nextmd]].js

[frouo]

96f54e52780e21b0f88ee067fcbe607815af11bd