Determine where prerelease docs go

With the new documentation site, we push to the latest branch with each release. This causes the docs to show up at /docs/latest/ (instead of /docs/head/).

When we do a prerelease, where do we want those docs to go? Maybe /docs/next/ so that /docs/latest/ preserves the docs for the latest official release?

This will need to be updated in Makefile.js.

[nzakas]

This will involve two steps:

1. Makefile.js needs to be updated to push to the “next” branch during prerelease instead of “latest”.
2. We need to update the docs site to use pathPrefix of “/docs/next/“ when deployed from the “next” branch. We also need the drop down to be updated to include “NEXT” beneath “HEAD”.

[snitin315]

@Gautam-Arora24 go ahead.