[Feature Request] Permalinks

[sjkim04]

While this is a simple option, I think permalinks should exist in mdBook.
It is very useful to almost everyone using mdBook, as this will lead to simplifying the URLs, deleting file extensions.
This is nearly identical to #1239, as permalinks will lead to clean URLs.

[ISSOtm]

Note that "permalinks" may already be handled by the server; for example, https://gbdev.io/pandocs/Rendering points to the same as https://gbdev.io/pandocs/Rendering.html. This is handled by GitHub Pages in our case, but it does beg the question of whether it's something that needs to be handled by mdBook itself in any way.