Build Warnings about Non-deterministic Routing

[chriswhong]

I believe this is related to mixing md and js files in /pages...

warn Non-deterministic routing danger: Attempting to create page: "/dev-404-page/", but page "/dev-404-page" already exists
This could lead to non-deterministic routing behavior
warn Non-deterministic routing danger: Attempting to create page: "/404/", but page "/404" already exists
This could lead to non-deterministic routing behavior
warn Non-deterministic routing danger: Attempting to create page: "/about/", but page "/about" already exists
This could lead to non-deterministic routing behavior
warn Non-deterministic routing danger: Attempting to create page: "/beta/", but page "/beta" already exists
This could lead to non-deterministic routing behavior
warn Non-deterministic routing danger: Attempting to create page: "/contact/", but page "/contact" already exists